Advertisement

Obstetric and perinatal outcomes in singleton pregnancies resulting from the transfer of frozen thawed versus fresh embryos generated through in vitro fertilization treatment: a systematic review and meta-analysis

      Objective

      To perform a systematic review and meta-analysis of obstetric and perinatal complications in singleton pregnancies after the transfer of frozen thawed and fresh embryos generated through IVF.

      Design

      Systematic review.

      Setting

      Observational studies, comparing obstetric and perinatal outcomes in singleton pregnancies subsequent to frozen thawed ET versus fresh embryo transfer, were included from Medline, EMBASE, Cochrane Central Register of Clinical Trials, DARE, and CINAHL (1984–2012).

      Patient(s)

      Women undergoing IVF/intracytoplasmic sperm injection (ICSI).

      Intervention(s)

      Two independent reviewers extracted data and assessed the methodological quality of the relevant studies using critical appraisal skills program scoring. Risk ratios and risk differences were calculated in Rev Man 5.1. Subgroup analysis was performed on matched cohort studies.

      Main Outcome Measure(s)

      Antepartum hemorrhage, very preterm birth, preterm birth, small for gestational age, low birth weight, very low birth weight, cesarean section, congenital anomalies, perinatal mortality, and admission to neonatal intensive care unit.

      Result(s)

      Eleven studies met the inclusion criteria. Singleton pregnancies after the transfer of frozen thawed embryos were associated with better perinatal outcomes compared with those after fresh IVF embryos. The relative risks (RR) and 95% confidence intervals (CI) of antepartum hemorrhage (RR = 0.67, 95% CI 0.55–0.81), preterm birth (RR = 0.84, 95% CI 0.78–0.90), small for gestational age (RR = 0.45, 95% CI 0.30–0.66), low birth weight (RR = 0.69, 95% CI 0.62–0.76), and perinatal mortality (RR = 0.68, 95% CI 0.48–0.96) were lower in women who received frozen embryos.

      Conclusion(s)

      Although fresh ET is the norm in IVF, results of this systematic review of observational studies suggest that pregnancies arising from the transfer of frozen thawed IVF embryos seem to have better obstetric and perinatal outcomes.

      Key Words

       Discuss

      You can discuss this article with its authors and with other ASRM members at http://fertstertforum.com/maheshwaria-obstetric-perinatal-outcomes-frozen-replacement-cycles-ivf/
      The first live birth after the transfer of a thawed cryopreserved embryo was reported in 1984. With refinement of technology during the past few decades, the proportion of frozen ETs has increased (
      • De Mouzon J.
      • Goossens V.
      • Bhattacharya S.
      • Castilla J.A.
      • Ferraretti A.P.
      • Korsak V.
      • et al.
      Assisted reproductive technology in Europe, 2006: results generated from European registers by ESHRE.
      ) as have pregnancy rates (PR) that, according to some investigators are better than those after the transfer of fresh IVF embryos (
      • Zhu D.
      • Zhang J.
      • Cao S.
      • Zhang J.
      • Heng B.C.
      • Huang M.
      • et al.
      Vitrified-warmed blastocyst transfer cycles yield higher pregnancy and implantation rates compared with fresh blastocyst transfer cycles—time for a new embryo transfer strategy?.
      ). Although there have been concerns about the impact of cryopreservation on the health of children born, outcome data after frozen ET have been generally reassuring (
      • Kansal Kalra S.
      • Ratcliffe S.J.
      • Milman L.
      • Gracia C.R.
      • Coutifaris C.
      • Barnhart K.T.
      Perinatal morbidity after in vitro fertilization is lower with frozen embryo transfer.
      ). Results of a number of observational studies (
      • Wada I.
      • Macnamee M.C.
      • Wick K.
      • Bradfield J.M.
      • Brinsden P.R.
      Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
      ,
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ,
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ) indicate that obstetric and perinatal outcomes in pregnancies resulting from cryopreserved embryos are similar to those in fresh IVF/intracytoplasmic sperm injection (ICSI) cycles, whereas other investigators suggest that they are superior (
      • Wang Y.A.
      • Sullivan E.A.
      • Black D.
      • Dean J.
      • Bryant J.
      • Chapman M.
      Preterm birth and low birth weight after assisted reproductive technology-related pregnancy in Australia between 1996 and 2000.
      ,
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ). Currently, there are no randomized controlled trials that have compared obstetric and perinatal outcomes after elective cryopreservation of all embryos followed by delayed transfer versus fresh ET in an IVF/ICSI cycle, which is the current practice in almost all IVF units. Individual observational studies, although large, lack the power to demonstrate clinically meaningful differences in key perinatal outcomes as the background risk of complications is very low.
      It is already known that obstetric and perinatal outcomes in singleton IVF/ICSI pregnancies are worse when compared with those after spontaneous conceptions (
      • Helmerhorst F.M.
      • Perquin D.A.M.
      • Donker D.
      • Keirse M.J.N.C.
      Perinatal outcome of singletons and twins after assisted conception: a systematic review of controlled studies.
      ). An early review (
      • Wennerholm U.B.
      • SoderstromAnttila V.
      • Bergh C.
      • Aittomaki K.
      • Hazekamp J.
      • Nygren K.G.
      • et al.
      Children born after cryopreservation of embryos or oocytes: a systematic review of outcome data.
      ) suggested that there was reduced incidence of preterm and low birth weight babies in pregnancies after frozen thawed ET. However, they did not attempt meta-analysis. Hence, it was not possible to quantify the obstetric and perinatal risks in singleton pregnancies after frozen thawed transfer.
      During the past few years an increased proportion of treatments in IVF/ICSI are after frozen ET (
      • De Mouzon J.
      • Goossens V.
      • Bhattacharya S.
      • Castilla J.A.
      • Ferraretti A.P.
      • Korsak V.
      • et al.
      Assisted reproductive technology in Europe, 2006: results generated from European registers by ESHRE.
      ). Furthermore, there are new technologies for freezing with higher survival rate of embryos. More data on outcomes are now available (at least six publications since last review). Hence, there is a need to perform a meta-analysis to quantify the risks of obstetric and perinatal complications in singleton IVF/ICSI pregnancies after frozen thawed ET and compare them with pregnancies after fresh ET.

      Materials and methods

      PRISMA guidelines for systematic reviews were followed (http://www.plosmedicine.org/article/info:doi%2F10.1371%2Fjournal.pmed.1000097).

       Data Sources and Searches

      A literature search was performed (1984–2012) on Medline, EMBASE, Cochrane Central Register of Clinical Trials, CINAHL, and DARE. There were no language restrictions. Relevant journals in the specialty (Human Reproduction, Human Reproduction Update, RBM online and Fertility and Sterility) were searched electronically. Cross references from the included studies were hand searched. Two review authors (A.M., S.P.) independently conducted the searches and selected the studies to be included. Differences of opinion were resolved after team discussion. Contact with authors was attempted wherever additional information was needed. Data were extracted using predesigned tables.

       Study Selection

       Inclusion criteria

      An initial scoping exercise did not reveal any randomized controlled trials. Hence all observational studies (published) that compared obstetric and perinatal outcomes in singleton pregnancies after transfer of fresh and frozen embryos were included.

       Exclusion criteria

      Studies were excluded if there was no comparator group, obstetric and perinatal outcomes were not reported, or if it was not possible to differentiate the outcomes for singletons and twins. Case reports and case series were excluded. Data from pregnancies after GIFT were excluded.

       Outcome Measures

      The following outcome measures were included: antepartum hemorrhage; hypertensive disorders of pregnancy (including pregnancy-induced hypertension, pre-eclampsia, and ecla-mpsia), gestational diabetes, very preterm birth (delivery before 32 weeks); preterm birth (delivery before 37 weeks); small for gestational age, low birth weight (birth weight <2,500 g); very low birth weight (birth weight <1,500 g); induction of labor; cesarean section (both emergency and elective); congenital anomalies (major and minor); perinatal mortality; and admission to the neonatal intensive care unit.

       Statistical Analysis

      For each outcome, data were extracted in 2 × 2 tables. If categorical data were not provided (to construct 2 × 2 tables) contact with investigators was attempted. Data were pooled if there were at least two studies with similar outcomes for the comparison groups. Meta-analysis was attempted wherever appropriate. Analysis was done using Rev Man 5.1 software (The Nordic Cochrane Centre, The Cochrane Collaboration 2011). For binary (or dichotomous) outcomes, the results for each study were expressed as risk ratios and risk differences with 95% confidence intervals (CI). Subgroup analysis was performed only on matched cohort studies.
      Quality assessment of included studies was performed independently by two authors (S.P. and A.M.). Any disagreement regarding type and quality of the study was resolved after discussion. Checklists from the critical appraisal skills program (CASP) (http://www.phru.nhs.uk/pages/phd/resources.htm) were used to assess and assign a quality score.

       Assessment of Heterogeneity

      We assessed whether there was sufficient similarity between the eligible studies in terms of design and clinical characteristics to allow pooling of data. Statistical heterogeneity was assessed by using the χ2 test. A low P value (or a large χ2 statistic relative to its degree of freedom) suggested heterogeneity (

      Higgins JPT GS, ed. Cochrane handbook for systematic reviews of interventions. 5.1st ed. Chichester, UK: John Wiley and Sons, 2011.

      ). The I2 statistic was used to assess the impact of the heterogeneity on the meta-analysis. In the event of moderate heterogeneity (I2 >50%), a sensitivity analysis was performed by altering the fixed-to-random effect analysis. Further sensitivity analyses were performed by excluding studies with low CASP score (CASP ≤10).

       Assessment of Reporting Biases

      Funnel plots were constructed where a statistically significant difference was obtained in outcome measure, if at least five studies reported that outcome. This was to investigate whether the difference was due to publication or reporting bias.

      Results

       Results of the Searches

      The literature search yielded 1,929 citations. Of these, 1,913 were excluded after reading the title and the abstract. Full text versions of 16 articles were obtained, whereas another 3 were identified from a hand search of cross references. Of these 19 articles, 11 were included in the completed review (Fig. 1). Table 1 gives the details of all included studies. The excluded studies, along with the reasons for exclusion, are provided in Table 2. Most studies were removed at the title and abstract stage as they did not give any data on outcomes after frozen replacement cycles.
      Figure thumbnail gr1
      Figure 1Selection of studies included in the review.
      Table 1Included studies.
      Study IDDesign of studyPopulationMethod of data collectionDetails of cryopreservationRisk of biasScoring
      Aflatoonian et al., 2010
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      Un matched cohort studyPregnancies after fresh ET vs. frozen ETQuestionnaires filled by gynecologists, pediatricians, and women regarding perinatal and obstetric outcomesVitrification was done at cleavage stage. Embryos were transferred in hormone replacement cyclesThe characteristics of two groups are similar. All pregnancies were included10/12
      Belva et al., 2008
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      Unmatched cohort studyIVF or ICSI cryo vs. fresh IVF or ICSI. Exposed cohort: all pregnancies irrespective of cryo procedure used were consecutively included. Unexposed cohort: fresh IVF/ICSI cyclesData on pregnancies, deliveries, and neonatal history were obtained by gynecologists, pediatricians, and double-checked with parents, when child was 2 months oldEmbryos were frozen at cleavage as well as blastocyst stage by slow freezing. Frozen embryos were transferred using natural cycles mainly but some were hormone replacement cyclePregnancies after mixed IVF/ICSI were not included. Cycles with PGD were excluded. Fresh cycles had maternal age—higher than frozen transfer. All pregnancies were included10/12
      Healy et al., 2010
      • Healy D.L.
      • Breheny S.
      • Halliday J.
      • Jaques A.
      • Rushford D.
      • Garrett C.
      • et al.
      Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
      Retrospective unmatched cohort study Jan 1991–Dec 2004Only fresh vs. frozen comparison was a subgroup analysisData were collected using record linkageNo details of frozen ET given natural/hormone replacement; cleavage/blastocyst; slow freezing/vitrificationIncludes first singleton birth only.

      Only ongoing pregnancies beyond 20 weeks of gestation were included. No data on demographic profile of the women in frozen ET vs. fresh group as this was subgroup analysis
      12/12
      Henningsen et al., 2011
      • Henningsen A.-A.
      • Pinborg A.
      • Lidegaard O.
      • Vestergaard C.
      • Forman J.L.
      • Andersen A.N.
      Perinatal outcome of singleton siblings born after assisted reproductive technology and spontaneous conception: Danish national sibling cohort study.
      Matched cohort with women acting as their own referenceData collection from national population-based registry from 1994–2008. Comparison groups: frozen vs. fresh IVF/ICSIThe data were collected from Danish Medical Birth RegisterNo details of embryo freezing protocol; stage of freezing and protocols for replacementWomen were their own reference.

      Fresh IVF/ICSI conception could be first/second one. Minimum gestation of included ongoing pregnancy was not mentioned
      11/12
      Pelkonen et al., 2010
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      Unmatched cohort study (1995–2006)Exposed cohort: frozen ET resulting in singleton pregnancy.

      Controls = fresh IVF/ICSI treatment. Some women may have had both fresh and frozen births; however, their proportion was <10%
      Data taken from Finnish Medical Birth RegisterBoth natural and hormone replacement frozen replacement cycles were included. Exact proportion not mentioned. Only slow freezing at cleavage stage was doneAll live births and stillbirths after 22 weeks of gestation and birth weight of ≥500 g were included. Mothers in frozen ET group were slightly older.

      Proportion of women having first pregnancy was 35% in frozen ET group compared with 52% in fresh ET group. The data on variables of pregnancy complications are incomplete in Finnish Medical Birth Register before 2004
      11.5/12
      Pinborg et al., 2010
      • Pinborg A.
      • Loft A.
      • Henningsen A.-A.
      • Rasmussen S.
      • Andersen A.N.
      Infant outcome of 957 singletons born after frozen embryo replacement: the Danish National Cohort Study 1995–2006.
      Matched cohort studyExposed cohort: singletons born after frozen ET (Jan 1995–Dec 2006). Unexposed cohort: singletons born after fresh IVF/ICSI within the same time frame 5 times the size of cryo groupDanish IVF and Danish Birth RegisterNo details of embryo freezing protocol; stage of freezing and protocols for replacementAge and parity showed statistically significant difference in the groups; data adjusted for age, parity, gender, and year of birth. Minimum gestation of included ongoing pregnancy was not mentioned11/12
      Shih et al., 2008
      • Shih W.
      • Rushford D.D.
      • Bourne H.
      • Garrett C.
      • McBain J.C.
      • Healy D.L.
      • et al.
      Factors affecting low birth weight after assisted reproduction technology: difference between transfer of fresh and cryopreserved embryos suggests an adverse effect of oocyte collection.
      Matched cohort with women acting as their own referenceComparison groups: frozen vs. fresh IVF/ICSINeonatal perinatal statistics unit AustraliaNo details of embryo freezing protocol; embryos frozen at cleavage stage protocols for replacement cycles were not specifiedAll pregnancies after 20 weeks were recorded. Fresh IVF/ICSI conception could be first/second one11/12
      Wada et al., 1994
      • Wada I.
      • Macnamee M.C.
      • Wick K.
      • Bradfield J.M.
      • Brinsden P.R.
      Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
      Unmatched cohort232 consecutive deliveries after embryo cryopreservation between 1985 and 1991. Fresh IVF data: 763, gestational age, birth weight, congenital abnormalitiesNot mentioned how data were collectedEmbryos frozen at early cleavage stage using slow freezingAll deliveries were included8/12
      Wang et al., 2005
      • Wang Y.A.
      • Sullivan E.A.
      • Black D.
      • Dean J.
      • Bryant J.
      • Chapman M.
      Preterm birth and low birth weight after assisted reproductive technology-related pregnancy in Australia between 1996 and 2000.
      Unmatched cohort studyInfants conceived through ART procedures and born in Australia during 1996–2000. Only the comparison between fresh and frozen singleton was used for this study (subgroup analysis from the main study)The study used data from two national collections. Assisted conception data collection & Australian national perinatal data collectionNo details of frozen ET given; natural/hormone replacement; cleavage/blastocyst; slow freezing/vitrificationFresh and frozen pregnancies were subgroup analysis, and hence not matched for the confounders. Viable pregnancy was defined as pregnancy of at least 20 weeks' gestation and or 400 g birth weight10/12
      Wennerholm et al., 1997
      • Wennerholm U.
      • Hamberger L.
      • Nilsson L.
      • Wennergren M.
      • Wikland M.
      • Bergh C.
      Obstetric and perinatal outcome of children conceived from cryopreserved embryos.
      Matched cohortUnexposed cohort: IVF conception with fresh embryos between 1990 and 1995 with frozen embryos.

      Exposed cohort: births between 1990 and 1995 with frozen embryos
      Data were collected after medical records reviewNo details of embryo freezing protocol; stage of freezing and protocols for replacementIVF conception and spontaneous pregnancies, both were control groups for pregnancies after frozen ET during that time period. Controls were matched for age and parity. All births were >28 weeks of gestation10.5/11
      Wikland et al., 2010
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      Unmatched cohort studyUnexposed cohort: fresh blastocyst transfer. Exposed cohort: pregnancies after transfer of vitrified, blastocystData for obstetric and perinatal complications were collected from maternity recordsBoth natural and hormone replacement frozen cycles were included; majority blastocyst cycles were hormonally supplementedFresh vs. frozen blastocysts only, although no matching was done but the characteristics were similar in the two groups. For the study period, deliveries in Sweden included all live born babies and stillbirths after 28 weeks of gestation. Maternal characteristics were comparable in vitrified and fresh blastocysts group concerning maternal age, BMI, parity, smoking, and education level11/12
      Note: BMI = body mass index; ICSI = intracytoplasmic sperm injection; PGD = preimplantation genetic diagnosis.
      Table 2Excluded studies.
      StudyReason for exclusion
      Aytoz et al., 1999
      • Aytoz A.
      • van Den Abbeel E.
      • Bonduelle M.
      • Camus M.
      • Joris H.
      • van Steirteghem A.
      • et al.
      Obstetric outcome of pregnancies after the transfer of cryopreserved and fresh embryos obtained by conventional in-vitro fertilization and intracytoplasmic sperm injection.
      Data from singleton and twins could not be separated
      Aflatoonian et al., 2010
      • Aflatoonian A.
      • Oskouian H.
      • Ahmadi S.
      • Oskouian L.
      Can fresh embryo transfers be replaced by cryopreserved-thawed embryo transfers in assisted reproductive cycles? A randomized controlled trial.
      No data on obstetric and perinatal outcomes
      Frydman et al., 1989

      Frydman R, Forman RG, Belaisch-Allart J, Hazout A, Fernandez H, Testart J. An obstetric analysis of fifty consecutive pregnancies after transfer of cryopreserved human embryos. Am J Obstet Gynecol;160:209–13.

      There is no control group
      Kallen et al., 2005
      • Kallen B.
      • Finnstrom O.
      • Nygren K.G.
      • Olausson P.O.
      In vitro fertilization (IVF) in Sweden: infant outcome after different IVF fertilization methods.
      2 × 2 table cannot be made
      Kallen et al., 2005
      • Kallen B.
      • Finnstrom O.
      • Nygren K.G.
      • Olausson P.O.
      In vitro fertilization (IVF) in Sweden: risk for congenital malformations after different IVF methods.
      Data for singleton cannot be separated
      Shapiro et al., 2011
      • Shapiro B.S.
      • Daneshmand S.T.
      • Garner F.C.
      • Aguirre M.
      • Hudson C.
      • Thomas S.
      Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: A prospective randomized trial comparing fresh and frozen-thawed embryo transfer in normal responders.
      No data on obstetric and perinatal outcomes
      Wennerholm, 2000
      • Wennerholm W.B.
      Cryopreservation of embryos and oocytes: obstetric outcome and health in children.
      Overlapping data from Wennerholm et al., 1997
      Wennerholm et al., 2009
      • Wennerholm U.B.
      • SoderstromAnttila V.
      • Bergh C.
      • Aittomaki K.
      • Hazekamp J.
      • Nygren K.G.
      • et al.
      Children born after cryopreservation of embryos or oocytes: a systematic review of outcome data.
      Systematic review

       Methodology of Included Studies

      Characteristics of the 11 included studies are provided in Table 1. Of 11 included publications 4 were matched cohort studies. All except one (
      • Wada I.
      • Macnamee M.C.
      • Wick K.
      • Bradfield J.M.
      • Brinsden P.R.
      Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
      ) scored high (≥10) on the CASP scoring system. Data were obtained from databases and data linkage of routinely collected data except in two studies where clinical information was reported by clinicians using case notes (
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ,
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ).

       Population in the Included Studies

      Although pregnancies conceived through IVF/ICSI using fresh or frozen embryos were included, there was a variation in the gestation after which they were included in the studies: all clinical pregnancies (
      • Wada I.
      • Macnamee M.C.
      • Wick K.
      • Bradfield J.M.
      • Brinsden P.R.
      Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
      ,
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ,
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ); ongoing pregnancies beyond 20 weeks (
      • Healy D.L.
      • Breheny S.
      • Halliday J.
      • Jaques A.
      • Rushford D.
      • Garrett C.
      • et al.
      Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
      ,
      • Shih W.
      • Rushford D.D.
      • Bourne H.
      • Garrett C.
      • McBain J.C.
      • Healy D.L.
      • et al.
      Factors affecting low birth weight after assisted reproduction technology: difference between transfer of fresh and cryopreserved embryos suggests an adverse effect of oocyte collection.
      ); beyond 22 weeks (
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ); and beyond 28 weeks (
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ,
      • Wennerholm U.
      • Hamberger L.
      • Nilsson L.
      • Wennergren M.
      • Wikland M.
      • Bergh C.
      Obstetric and perinatal outcome of children conceived from cryopreserved embryos.
      ).
      In two studies women acted as their own reference group (
      • Shih W.
      • Rushford D.D.
      • Bourne H.
      • Garrett C.
      • McBain J.C.
      • Healy D.L.
      • et al.
      Factors affecting low birth weight after assisted reproduction technology: difference between transfer of fresh and cryopreserved embryos suggests an adverse effect of oocyte collection.
      ,
      • Henningsen A.-A.
      • Pinborg A.
      • Lidegaard O.
      • Vestergaard C.
      • Forman J.L.
      • Andersen A.N.
      Perinatal outcome of singleton siblings born after assisted reproductive technology and spontaneous conception: Danish national sibling cohort study.
      ). Fresh IVF/ICSI could have been first or second conception. Two studies had cohort group matched for age and parity (
      • Wennerholm U.
      • Hamberger L.
      • Nilsson L.
      • Wennergren M.
      • Wikland M.
      • Bergh C.
      Obstetric and perinatal outcome of children conceived from cryopreserved embryos.
      ,
      • Pinborg A.
      • Loft A.
      • Henningsen A.-A.
      • Rasmussen S.
      • Andersen A.N.
      Infant outcome of 957 singletons born after frozen embryo replacement: the Danish National Cohort Study 1995–2006.
      ). There was no information on demographic profiles of women in two other studies (
      • Wang Y.A.
      • Sullivan E.A.
      • Black D.
      • Dean J.
      • Bryant J.
      • Chapman M.
      Preterm birth and low birth weight after assisted reproductive technology-related pregnancy in Australia between 1996 and 2000.
      ,
      • Healy D.L.
      • Breheny S.
      • Halliday J.
      • Jaques A.
      • Rushford D.
      • Garrett C.
      • et al.
      Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
      ) as we used subgroup of fresh and frozen thawed treatment. Demographic information on these subgroups was not separate from the overall information. The characteristics in the two groups were similar in two unmatched cohort studies (
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ,
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ). Maternal age was higher in the fresh ET group in Belva et al. (
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ), whereas it was lower in Pelkonen et al. (
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ) when compared with the frozen ET group. No details on other confounders such as parity, smoking, duration of infertility, and pre-existing medical diseases were available.

       Exposure in the Included Studies

      In the included studies, embryos were frozen on day 2/3 (cleavage stage) or day 5/6 (blastocyst stage) using either vitrification (newer method) or slow freezing (older method) techniques. Frozen embryos (after thawing) were transferred with or without giving additional hormones to women (to prepare endometrium) in a menstrual cycle (Fig. 1).
      Six of 11 studies did not provide details of the method used for embryo cryopreservation, developmental stage of the embryo, and whether exogenous hormones were used to prepare the endometrium for frozen ET. Of those studies reporting these details, there was variation in the method of cryopreservation and the developmental stage of embryos at cryopreservation (Table 1). Most studies used slow freezing (
      • Wada I.
      • Macnamee M.C.
      • Wick K.
      • Bradfield J.M.
      • Brinsden P.R.
      Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
      ,
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ,
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ), whereas Wikland et al. (
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ) and Aflatoonian et al. (
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ) used vitrification. The embryos were frozen at cleavage stage (
      • Wada I.
      • Macnamee M.C.
      • Wick K.
      • Bradfield J.M.
      • Brinsden P.R.
      Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
      ,
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ,
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ), at blastocyst stage (
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ), and at blastocyst as well as cleavage stage (
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ). Embryos were transferred either in hormone replacement cycles (
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ) or natural, as well as hormone replacement cycles (
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ,
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ,
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ).

       Results of the Outcome Measures

       Antepartum hemorrhage

      Two unmatched cohort studies were included in the meta-analysis (n = 3,875 vs. 7,000 pregnancies after frozen vs. fresh ET) (
      • Healy D.L.
      • Breheny S.
      • Halliday J.
      • Jaques A.
      • Rushford D.
      • Garrett C.
      • et al.
      Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
      ,
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ). Antepartum hemorrhage was defined as bleeding of 15 mL or more that occurred from birth canal after 20 weeks of gestation and before the birth of the baby (
      • Healy D.L.
      • Breheny S.
      • Halliday J.
      • Jaques A.
      • Rushford D.
      • Garrett C.
      • et al.
      Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
      ). Placenta previa and placenta abruptio are reported separately by both studies. Singleton pregnancies after frozen thawed ET were at decreased risk of antepartum hemorrhage when compared with those after fresh embryos (relative risk [RR] = 0.67, 95% CI 0.55–0.81) with an absolute decrease in the risk of 2% (1%–2%). There was no heterogeneity among the studies.
      There was a lower risk of placenta previa (RR = 0.71, 95% CI 0.53–0.95, I2 = 0) and placental abruptio (RR = 0.44, 95% CI 0.24–0.83, I2 = 0) in pregnancies after frozen thawed ETs (Supplemental Fig. 1, available online).

       Very preterm birth (delivery at <32 weeks)

      Of four studies (n = 3,050 vs. 13,630 pregnancies after frozen vs. fresh ET) reporting proportion of deliveries at <32 weeks, two were case control studies. All studies had high score on CASP scoring. The relative risk (95% CI) of having delivery at <32 weeks was 0.73 (0.50–1.08) in singleton pregnancies after frozen embryo thawed transfer when compared with those after fresh embryo transfer. There was minimal heterogeneity (I2 = 11%) among the studies.

       Preterm delivery (delivery at <37 weeks)

      Of nine studies (n = 10,017 vs. 27,686 pregnancies after frozen vs. fresh cycles) reporting proportion of deliveries at <37 weeks, three were matched cohort. All were good quality studies as per CASP scoring. Definition of preterm labor/delivery was delivery before 37 weeks in all studies. There were no data on how many of them were spontaneous or induced preterm labor.
      The RR of having a delivery at <37 weeks was 0.84 (95% CI 0.78–0.90) in singleton pregnancies after frozen thawed ET, when compared with those after fresh ETs (Supplemental Fig. 2, available online) with an absolute decrease in risk (95% CI) of 2% (1%–3%). There was marked heterogeneity (I2 = 74%) among the studies. The reduction in the risk persisted when subgroup analysis was done using only matched cohort and good quality studies, but not when the analysis was repeated with random effect. Funnel plot did not reveal any publication bias (Supplemental Fig. 3, available online).

       Small for gestational age

      Two studies (n = 1,933 vs. 3,141 pregnancies after frozen vs. fresh cycles) have reported on outcome of small for gestational age. This was defined as birth weight <22% of the expected mean birth weight according to gestational age in a Swedish reference population (
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ) or birth weight <2 SD of the mean for that gestation (
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ).
      The RR of having a small for gestational age baby was 0.45 (95% CI 0.30–0.66) in singleton pregnancies subsequent to frozen thawed ET compared with those with fresh ET (Supplemental Fig. 4, available online) with an absolute decrease in risk (95% CI) of 2% (1%–2%). There was minimal heterogeneity among the studies (I2 = 22%).

       Low birth weight (birth weight <2,500 g)

      Of nine studies (n = 8,536 vs. 25,800 pregnancies after frozen vs. fresh cycles) reporting this outcome measure three were matched cohort (Supplemental Fig. 5, available online). Eight studies had high CASP scores. The RR of having a baby with birth weight <2,500 g was 0.69 (95% CI 0.62–0.76) in singleton pregnancies after frozen thawed embryos when compared with those after fresh embryos with an absolute decrease in risk (95% CI) of 3% (2%–3%). There was minimal heterogeneity (I2 = 28%) among the studies. The increased risk persisted when data were only pooled from matched cohort and good quality studies (Table 3). Funnel plot did not reveal any publication bias (Supplemental Fig. 6, available online).
      Table 3Overall table for effect and sensitivity analysis (frozen vs. fresh IVF/ICSI pregnancies).
      OutcomeNo. of frozen vs. fresh IVF/ICSI conceptionsOverall effect (RR, 95% CI) fixed effectHeterogeneity (I2)Subgroup analysis (matched cohort) (calculated only if there was a statistical difference)Risk difference (calculated only if there was a statistical difference)Sensitivity analysis
      Sensitivity analysis performed only if there was a mixture of good and poor quality studies.


      Good quality studies only (CASP>10)
      Small for gestational age1,933 vs. 3,1410.45 (0.30–0.66)22%NA−0.02 (−0.03, −0.01)
      Birth weight <2,500 g8,536 vs. 25,8000.69 (0.62–0.76)28%0.59 (0.45–0.78)−0.03 (−0.03, −0.02)0.69 (0.63, −0.76)
      Birth weight <1,500 g3,552 vs. 16,4690.72 (0.50–1.04)0NANA
      Delivery at <37 weeks10,017 vs. 27,6860.84 (0.78–0.90)
      Random effect model −0.86 (0.72–1.03).
      74%0.72 (0.63–0.82)−0.02 (−0.03, −0.01)
      Delivery at <32 weeks3,050 vs. 13,6300.73 (0.50–1.08)11%0.76 (0.44–1.33)−0.00 (−0.01–0.00)
      APH3,875 vs. 7,0000.67 (0.55–0.81)0NA−0.02 (−0.02, −0.01)
      Congenital anomalies3,152 vs. 6,3081.05 (0.81–1.35)47%0.88 (0.63–1.24)NA
      Cesarean section5,435 vs. 16,7401.10 (1.05–1.15)18%1.12 (1.06–1.18)0.03 (0.01, −0.05)
      Transfer to NICU3,552 vs. 16,4691.00 (0.92–1.08)69%0.86 (0.72–1.03)NA
      Perinatal mortality5,546 vs. 17,4240.68 (0.48–0.96)00.64 (0.43–0.97)−0.00 (−0.01, −0.00)0.64 (0.43, −0.97)
      Note: APH = antepartum hemorrhage; CASP = critical appraisal skills program; CI = confidence interval; NA = not available; NICU = neonatal intensive care unit; RR = risk ratio.
      a Sensitivity analysis performed only if there was a mixture of good and poor quality studies.
      b Random effect model −0.86 (0.72–1.03).

       Very low birth weight (birth weight <1,500 g)

      Of four studies (n = 3,552 vs. 16,469 pregnancies after frozen vs. fresh ET) reporting proportion of deliveries with birth weight <1,500 g, one was a matched cohort. All studies had high CASP scores.
      The RR of having a baby with birth weight <1,500 g was 0.72 (95% CI 0.50–1.04) after singleton pregnancies subsequent to frozen thawed ET compared with those after fresh ET. There was no heterogeneity (I2 = 0) among the studies.

       Cesarean section

      Five studies (3 matched cohort) were included in the meta-analysis (n = 5,435 vs. 16,740 pregnancies after fresh vs. frozen ET). Only two studies (
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ,
      • Wennerholm U.
      • Hamberger L.
      • Nilsson L.
      • Wennergren M.
      • Wikland M.
      • Bergh C.
      Obstetric and perinatal outcome of children conceived from cryopreserved embryos.
      ) gave separate data on elective and emergency cesarean section. Elective and emergency cesarean sections were pooled together (proportion of elective and emergency cesarean section was equal in both groups). Pregnancies after frozen thawed embryos had an increased risk of cesarean section when compared with those after fresh embryos (RR = 1.10, 95% CI 1.05–1.15) with an absolute risk increase of 3% (1%–5%). There was low heterogeneity (I2 = 18%) among the studies (Supplemental Fig. 7, available online). Subgroup analysis did not alter the results (Table 3). Funnel plot did not reveal any publication bias (Supplemental Fig. 8).

       Congenital anomalies

      Only three studies (n = 3,152 vs. 6,308 pregnancies after frozen vs. fresh ET) reported congenital anomalies (one matched cohort study). Major and minor anomalies were pooled together. The RR of having a congenital anomaly was 1.05 (95% CI 0.81–1.35) in pregnancies after frozen thawed embryos compared with fresh embryos. There was moderate heterogeneity (I2 = 47%) among the studies.

       Perinatal mortality

      Of six studies (n = 5,546 vs. 17,424 pregnancies after frozen vs. fresh ET) reporting perinatal mortality, three were matched cohort. There was a slight variation in the definition of perinatal mortality: death of child with a gestational age of >20 weeks of gestation or up to day 28 of birth (
      • Aflatoonian A.
      • Mansoori Moghaddam F.
      • Mashayekhy M.
      • Mohamadian F.
      Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
      ,
      • Belva F.
      • Henriet S.
      • van den Abbeel E.
      • Camus M.
      • Devroey P.
      • van der Elst J.
      • et al.
      Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
      ); deaths occurring after the 24th week of gestation and during the first week of life (
      • Wada I.
      • Macnamee M.C.
      • Wick K.
      • Bradfield J.M.
      • Brinsden P.R.
      Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
      ); after 22 weeks of gestation and first 7 days of life (
      • Pelkonen S.
      • Koivunen R.
      • Gissler M.
      • Nuojua-Huttunen S.
      • Suikkari A.
      • Hyden-Granskog C.
      • et al.
      Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
      ); stillbirth after 28 weeks of gestation and first 7 days of life (
      • Wikland M.
      • Hardarson T.
      • Hillensjo T.
      • Westin C.
      • Westlander G.
      • Wood M.
      • et al.
      Obstetric outcomes after transfer of vitrified blastocysts.
      ); stillbirth after 20 weeks, later terminations and all neonatal deaths (
      • Shih W.
      • Rushford D.D.
      • Bourne H.
      • Garrett C.
      • McBain J.C.
      • Healy D.L.
      • et al.
      Factors affecting low birth weight after assisted reproduction technology: difference between transfer of fresh and cryopreserved embryos suggests an adverse effect of oocyte collection.
      ).
      The RR of perinatal mortality was 0.68 (95% CI 0.48–0.96) in singleton pregnancies after frozen thawed ETs when compared with those after fresh embryos (Supplemental Fig. 9, available online). The difference persisted in subgroup analysis of only matched cohort studies (RR = 0.64, 95% CI 0.43–0.97). There was no heterogeneity among the studies (I2 = 0).

       Admission to neonatal intensive care unit

      Three unmatched and one matched cohort study reported the outcome of admission to the neonatal intensive care unit (n = 3,552 vs. 16,469 pregnancies after frozen vs. fresh ET). The length and the reasons for neonatal intensive care unit admission were not specified. There was no increase in the risk of admission to the neonatal intensive care unit (RR = 1.00, 95% CI 0.92–1.08) in pregnancies after frozen embryos. There was marked heterogeneity among the studies (I2 = 69%).
      Hypertensive disorders of pregnancy, gestational diabetes, preterm premature rupture of membranes, and induction of labor was not reported by any of the included studies. These can only be answered by individual patient data meta-analysis.

      Discussion

       Main Findings

      Singleton pregnancies after transfer of frozen thawed embryos were associated with a lower risk of perinatal mortality, small for gestational age baby, preterm birth (<37 weeks), low birth weight baby (<2,500 g), and antepartum hemorrhage when compared with those after fresh ETs. However, the risk of a baby being born by cesarean section was lower in pregnancies after fresh ET.

       Strengths

      This is the first systematic review and first meta-analysis on large data to quantify the obstetric and perinatal risks for singleton pregnancies after frozen ET compared with those after fresh ETs. Absolute risk differences were calculated in two groups to give a clearer idea to clinicians managing these women as opposed to RRs alone. Sensitivity analyses were performed to evaluate the robustness of the existing data.

       Limitations

      Because there are no randomized controlled trials, this review is limited to data from observational studies. Individual methodological differences, variation in design, inclusion exclusion criteria, definition, and ascertainment of outcomes are inherent in systematic reviews of observational studies. Patients who have had fresh cycle may be different from those who had frozen replacement cycles. It is likely that the fresh cycle population are made up of both good and bad prognosis patients, whereas the frozen embryo population are more likely composed of the better prognosis patients and hence the difference in the outcomes. We were unable to adjust for confounders such as age, smoking, parity, duration of infertility, and pre-existing medical illness due to varied design of the studies. There is clinical heterogeneity in terms of the population sampled, design of studies, method of freezing, and regimens in replacement cycles (Table 1). Without individual patient data, we are unable to adjust for confounders and determine whether the risks are different for embryos frozen by slow freezing and vitrification; embryos frozen at cleavage and blastocyst stages. When it comes to thawing the embryos, there is uncertainty as to whether method of thawing and protocol used (natural or hormonally mediated cycle) for replacement has any bearing on different obstetric and perinatal outcomes.
      Although data from this review suggest that both preterm labor and low birth weight are decreased in pregnancies subsequent to frozen ET, these two outcomes are related. The two studies reporting gestation-adjusted birth weights, however, report a decrease in small for gestational age babies in pregnancies subsequent to frozen ET. We could not differentiate between spontaneous and iatrogenic preterm delivery. There was no evidence of difference in neonatal intensive care unit admissions in both groups, which is not in keeping with the decreased risks of preterm and low birth weight reported with frozen ET.
      There is inconsistency in definition of outcomes, such as antepartum hemorrhage, congenital anomalies, and perinatal mortality, as has been highlighted. In addition, not all outcomes have been reported by all studies.

       Explanation of Results

      The reasons for better outcomes for frozen ET cycles compared with fresh ET are not known. It has been suggested that replacing embryos on the second or third day after fertilization in a fresh IVF cycle involves a degree of asynchrony between the phase of development of the embryo and endometrial receptivity (due to excessive estrogen [E] and P as a result of ovarian stimulation), which can affect implantation (
      • Shapiro B.S.
      • Daneshmand S.T.
      • Garner F.C.
      • Aguirre M.
      • Hudson C.
      • Thomas S.
      Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: A prospective randomized trial comparing fresh and frozen-thawed embryo transfer in normal responders.
      ) in fresh IVF/ICSI treatment cycles. It has been suggested that a more natural uterine environment that occurs in a frozen replacement cycle is favorable for early placentation and embryogenesis, whereas ovarian stimulation in fresh cycles alter endometrial angiogenesis and implantation (
      • Kansal Kalra S.
      • Ratcliffe S.J.
      • Milman L.
      • Gracia C.R.
      • Coutifaris C.
      • Barnhart K.T.
      Perinatal morbidity after in vitro fertilization is lower with frozen embryo transfer.
      ,
      • Healy D.L.
      • Breheny S.
      • Halliday J.
      • Jaques A.
      • Rushford D.
      • Garrett C.
      • et al.
      Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
      ,
      • Amor D.J.
      • Xu J.X.
      • Halliday J.L.
      • Francis I.
      • Healy D.L.
      • Breheny S.
      • et al.
      Pregnancies conceived using assisted reproductive technologies (ART) have low levels of pregnancy-associated plasma protein-A (PAPP-A) leading to a high rate of false-positive results in first trimester screening for Down syndrome.
      ). This argument is further strengthened by the findings of Healy et al. (
      • Healy D.L.
      • Breheny S.
      • Halliday J.
      • Jaques A.
      • Rushford D.
      • Garrett C.
      • et al.
      Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
      ), who showed that the increase in antepartum hemorrhage after fresh ETs increased as the number of oocytes increased (increased numbers of oocytes increases the concentration of E in circulation, hence exposing the endometrium to excess E).
      The studies included in this review did not differentiate between frozen ETs done in natural or hormone replacement cycles. However, it has been argued that even when hormone replacement cycles have been used in frozen ETs, E and P are given in physiological doses to mimic natural cycles, compared with stimulation of fresh cycles where supraphysiological doses of gonadotropins are given (
      • Shapiro B.S.
      • Daneshmand S.T.
      • Garner F.C.
      • Aguirre M.
      • Hudson C.
      • Thomas S.
      Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: A prospective randomized trial comparing fresh and frozen-thawed embryo transfer in normal responders.
      ,
      • Aflatoonian A.
      • Oskouian H.
      • Ahmadi S.
      • Oskouian L.
      Can fresh embryo transfers be replaced by cryopreserved-thawed embryo transfers in assisted reproductive cycles? A randomized controlled trial.
      ).
      Another explanation put forward for better results in pregnancies subsequent to frozen ET is that the physical effects of freezing and thawing embryos may filter out weaker embryos and allow only good quality ones to survive, resulting in better fetal growth (
      • Shih W.
      • Rushford D.D.
      • Bourne H.
      • Garrett C.
      • McBain J.C.
      • Healy D.L.
      • et al.
      Factors affecting low birth weight after assisted reproduction technology: difference between transfer of fresh and cryopreserved embryos suggests an adverse effect of oocyte collection.
      ).
      The high rate of cesarean section in pregnancies subsequent to frozen ET could be because these women may have had previous cesarean sections. In the subgroup analysis of studies where women were their own reference group, hence had previous IVF conception (which is more likely to have cesarean section), high rate of cesarean section was observed (Supplemental Fig. 7).

       Clinical Implications of the Study

      Although obstetric and perinatal risks are less in pregnancies subsequent to frozen thawed embryos, the absolute reduction in some of these risks, including perinatal mortality, which fortunately are rare, are likely to be small. However, data from this review provide reassurance for cryopreservation programs.
      In current practice, the best embryo is selected for transfer in the fresh cycle and spare ones are frozen (if they are deemed suitable to survive freezing thawing process). As our data suggest that pregnancies after frozen replacement cycles may have better obstetric and perinatal outcomes compared with those after fresh embryos, intuitively one would think that we should freeze all embryos and transfer them subsequently in what are considered as optimal conditions of endometrium for implantation. However, the better obstetric and perinatal outcomes presented in this review are per ongoing pregnancy. Despite improvement in cryopreservation, most places have better PRs with fresh treatment compared with frozen thawed replacement. Hence outcomes per started cycle may not be different and cost effective, especially when the costs of freeze and thaw cycles are added.
      There are suggestions that the endometrial development in frozen thawed cycles can be controlled more precisely than in the cycles of ovarian stimulation with gonadotropins (
      • Shapiro B.S.
      • Daneshmand S.T.
      • Garner F.C.
      • Aguirre M.
      • Hudson C.
      • Thomas S.
      Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: A prospective randomized trial comparing fresh and frozen-thawed embryo transfer in normal responders.
      ,
      • Aflatoonian A.
      • Oskouian H.
      • Ahmadi S.
      • Oskouian L.
      Can fresh embryo transfers be replaced by cryopreserved-thawed embryo transfers in assisted reproductive cycles? A randomized controlled trial.
      ). The segmentation (separation of ovarian stimulation and ET in different menstrual cycles) of IVF/ICSI treatment has also been suggested to reduce the risks of ovarian hyperstimulation syndrome (OHSS) (
      • Devroey P.
      • Polyzos N.P.
      • Blockeel C.
      An OHSS free clinic by segmentation of IVF treatment.
      ).
      Two recent small randomized trials have suggested that PRs after elective cryopreservation and later ET are at least similar to those after fresh ET in women who are predicted hyperresponders (i.e., those women who are at high risk of hyperstimulation) (
      • Aflatoonian A.
      • Oskouian H.
      • Ahmadi S.
      • Oskouian L.
      Can fresh embryo transfers be replaced by cryopreserved-thawed embryo transfers in assisted reproductive cycles? A randomized controlled trial.
      ), as well as those women who are normal responders (
      • Shapiro B.S.
      • Daneshmand S.T.
      • Garner F.C.
      • Aguirre M.
      • Hudson C.
      • Thomas S.
      Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: A prospective randomized trial comparing fresh and frozen-thawed embryo transfer in normal responders.
      ). As cryopreservation facilities and techniques improve further, it might be possible to introduce the practice of elective cryopreservation for later use.

       Future Research

      Randomized controlled trials are needed to evaluate clinical and cost effectiveness, as well as acceptability of the practice of elective cryopreservation versus current practice of fresh ET.
      In conclusion, singleton pregnancies after frozen thawed ET seem to have better outcomes than those after fresh ETs (which are the norm in IVF treatment). There is lower risk of perinatal mortality, small for gestational age, preterm birth, low birth weight, and antepartum hemorrhage. Further evidence is needed before elective cryopreservation can be routinely recommended in preference to current practice of fresh ET.

      Appendix

      Figure thumbnail fx1
      Supplemental Figure 1Antepartum hemorrhage. CI = confidence interval; M-H = Mantel-Haenszel.
      Figure thumbnail fx2
      Supplemental Figure 2Preterm labor. CI = confidence interval; M-H = Mantel-Haenszel.
      Figure thumbnail fx3
      Supplemental Figure 3Funnel plot for studies reporting preterm labor.
      Figure thumbnail fx4
      Supplemental Figure 4Small for gestational age. CI = confidence interval; M-H = Mantel-Haenszel.
      Figure thumbnail fx5
      Supplemental Figure 5Low birth weight (birth weight <2,500 g). CI = confidence interval; M-H = Mantel-Haenszel.
      Figure thumbnail fx6
      Supplemental Figure 6Funnel plot for studies reporting low birth weight.
      Figure thumbnail fx7
      Supplemental Figure 7Cesarean section. CI = confidence interval; M-H = Mantel-Haenszel.
      Figure thumbnail fx8
      Supplemental Figure 8Funnel plot for studies reporting on cesarean sections.
      Figure thumbnail fx9
      Supplemental Figure 9Perinatal mortality. CI = confidence interval; M-H = Mantel-Haenszel.

      References

        • De Mouzon J.
        • Goossens V.
        • Bhattacharya S.
        • Castilla J.A.
        • Ferraretti A.P.
        • Korsak V.
        • et al.
        Assisted reproductive technology in Europe, 2006: results generated from European registers by ESHRE.
        Hum Reprod. 2010; 25: 1851-1862
        • Zhu D.
        • Zhang J.
        • Cao S.
        • Zhang J.
        • Heng B.C.
        • Huang M.
        • et al.
        Vitrified-warmed blastocyst transfer cycles yield higher pregnancy and implantation rates compared with fresh blastocyst transfer cycles—time for a new embryo transfer strategy?.
        Fertil Steril. 2011; 95: 1691-1695
        • Kansal Kalra S.
        • Ratcliffe S.J.
        • Milman L.
        • Gracia C.R.
        • Coutifaris C.
        • Barnhart K.T.
        Perinatal morbidity after in vitro fertilization is lower with frozen embryo transfer.
        Fertil Steril. 2011; 95: 548-553
        • Wada I.
        • Macnamee M.C.
        • Wick K.
        • Bradfield J.M.
        • Brinsden P.R.
        Birth characteristics and perinatal outcome of babies conceived from cryopreserved embryos.
        Hum Reprod. 1994; 9: 543-546
        • Wikland M.
        • Hardarson T.
        • Hillensjo T.
        • Westin C.
        • Westlander G.
        • Wood M.
        • et al.
        Obstetric outcomes after transfer of vitrified blastocysts.
        Hum Reprod. 2010; 25: 1699-1707
        • Aflatoonian A.
        • Mansoori Moghaddam F.
        • Mashayekhy M.
        • Mohamadian F.
        Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles.
        J Assist Reprod Genet. 2010; 27: 695-700
        • Wang Y.A.
        • Sullivan E.A.
        • Black D.
        • Dean J.
        • Bryant J.
        • Chapman M.
        Preterm birth and low birth weight after assisted reproductive technology-related pregnancy in Australia between 1996 and 2000.
        Fertil Steril. 2005; 83: 1650-1658
        • Belva F.
        • Henriet S.
        • van den Abbeel E.
        • Camus M.
        • Devroey P.
        • van der Elst J.
        • et al.
        Neonatal outcome of 937 children born after transfer of cryopreserved embryos obtained by ICSI and IVF and comparison with outcome data of fresh ICSI and IVF cycles.
        Hum Reprod. 2008; 10: 2227-2238
        • Helmerhorst F.M.
        • Perquin D.A.M.
        • Donker D.
        • Keirse M.J.N.C.
        Perinatal outcome of singletons and twins after assisted conception: a systematic review of controlled studies.
        BMJ. 2004; 328: 261
        • Wennerholm U.B.
        • SoderstromAnttila V.
        • Bergh C.
        • Aittomaki K.
        • Hazekamp J.
        • Nygren K.G.
        • et al.
        Children born after cryopreservation of embryos or oocytes: a systematic review of outcome data.
        Hum Reprod. 2009; 24: 2158-2172
      1. Higgins JPT GS, ed. Cochrane handbook for systematic reviews of interventions. 5.1st ed. Chichester, UK: John Wiley and Sons, 2011.

        • Healy D.L.
        • Breheny S.
        • Halliday J.
        • Jaques A.
        • Rushford D.
        • Garrett C.
        • et al.
        Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria, Australia.
        Hum Reprod. 2010; 25: 265-274
        • Shih W.
        • Rushford D.D.
        • Bourne H.
        • Garrett C.
        • McBain J.C.
        • Healy D.L.
        • et al.
        Factors affecting low birth weight after assisted reproduction technology: difference between transfer of fresh and cryopreserved embryos suggests an adverse effect of oocyte collection.
        Hum Reprod. 2008; 23: 1644-1653
        • Pelkonen S.
        • Koivunen R.
        • Gissler M.
        • Nuojua-Huttunen S.
        • Suikkari A.
        • Hyden-Granskog C.
        • et al.
        Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995–2006.
        Hum Reprod. 2010; 25: 914-923
        • Wennerholm U.
        • Hamberger L.
        • Nilsson L.
        • Wennergren M.
        • Wikland M.
        • Bergh C.
        Obstetric and perinatal outcome of children conceived from cryopreserved embryos.
        Hum Reprod. 1997; 12: 1819-1825
        • Henningsen A.-A.
        • Pinborg A.
        • Lidegaard O.
        • Vestergaard C.
        • Forman J.L.
        • Andersen A.N.
        Perinatal outcome of singleton siblings born after assisted reproductive technology and spontaneous conception: Danish national sibling cohort study.
        Fertil Steril. 2011; 95: 959-963
        • Pinborg A.
        • Loft A.
        • Henningsen A.-A.
        • Rasmussen S.
        • Andersen A.N.
        Infant outcome of 957 singletons born after frozen embryo replacement: the Danish National Cohort Study 1995–2006.
        Fertil Steril. 2010; 94: 1320-1327
        • Shapiro B.S.
        • Daneshmand S.T.
        • Garner F.C.
        • Aguirre M.
        • Hudson C.
        • Thomas S.
        Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: A prospective randomized trial comparing fresh and frozen-thawed embryo transfer in normal responders.
        Fertil Steril. 2011; 96: 344-348
        • Amor D.J.
        • Xu J.X.
        • Halliday J.L.
        • Francis I.
        • Healy D.L.
        • Breheny S.
        • et al.
        Pregnancies conceived using assisted reproductive technologies (ART) have low levels of pregnancy-associated plasma protein-A (PAPP-A) leading to a high rate of false-positive results in first trimester screening for Down syndrome.
        Hum Reprod. 2009; 24: 1330-1338
        • Aflatoonian A.
        • Oskouian H.
        • Ahmadi S.
        • Oskouian L.
        Can fresh embryo transfers be replaced by cryopreserved-thawed embryo transfers in assisted reproductive cycles? A randomized controlled trial.
        J Assist Reprod Genet. 2010; 27: 357-363
        • Devroey P.
        • Polyzos N.P.
        • Blockeel C.
        An OHSS free clinic by segmentation of IVF treatment.
        Hum Reprod. 2011; 10: 2593-2597
        • Aytoz A.
        • van Den Abbeel E.
        • Bonduelle M.
        • Camus M.
        • Joris H.
        • van Steirteghem A.
        • et al.
        Obstetric outcome of pregnancies after the transfer of cryopreserved and fresh embryos obtained by conventional in-vitro fertilization and intracytoplasmic sperm injection.
        Hum Reprod. 1999; 14: 2619-2624
      2. Frydman R, Forman RG, Belaisch-Allart J, Hazout A, Fernandez H, Testart J. An obstetric analysis of fifty consecutive pregnancies after transfer of cryopreserved human embryos. Am J Obstet Gynecol;160:209–13.

        • Kallen B.
        • Finnstrom O.
        • Nygren K.G.
        • Olausson P.O.
        In vitro fertilization (IVF) in Sweden: infant outcome after different IVF fertilization methods.
        Fertil Steril. 2005; 84: 611-617
        • Kallen B.
        • Finnstrom O.
        • Nygren K.G.
        • Olausson P.O.
        In vitro fertilization (IVF) in Sweden: risk for congenital malformations after different IVF methods.
        Birth Defects Res. 2005; 73: 162-169
        • Wennerholm W.B.
        Cryopreservation of embryos and oocytes: obstetric outcome and health in children.
        Hum Reprod. 2000; 15: 18-25