Monozygotic twinning (MZT) rates increase with embryo cohort quality and are not directly related to cleavage (CL) stage or blastocyst (BL) stage transfer


      MZT rates are increased in ART pregnancies with many reporting the greatest prevalence following BL transfer. It is unknown whether this increase is attributable to extended culture or to intrinsic differences in the populations selected for CL vs BL transfer. This study evaluates the MZT rate by day of transfer after normalizing the populations being compared.



      Materials and Methods

      All autologous fresh IVF cycles resulting in clinical pregnancies were studied. MZT following CL and BL transfer were compared after controlling for prognostic factors and for the overall quality of the developing cohort of embryos.


      233 of 9272 clinical pregnancies (2.5%) had MZT. The prevalence was higher following BL than CL transfers (1.9% vs 3.0%, P=0.0005). After grouping by maternal age, BL and CL transfer had equivalent MZT. Controlling for ovarian response or embryonic cohort size also negated any differences. Grouping by mean cell number of the developing cohort of embryos on day 3 showed increasing MZT as the quality of the overall cohort improved (P<0.0001). Consistent with this, grouping by mean cell number of the entire cohort on day 3 led to equivalent MZT rates following CL or BL transfer. Similar analyses controlling for other markers of embryo cohort quality consistently showed equivalent MZT for CL and BL transfer. The highest risk for MZT was in those with supernumerary embryos for cryopreservation, reflecting good cohort quality. While high, the CL and BL transfers were equivalent. Two factors did not increase MZT rates: ICSI or transfer order.


      MZT was increased in patients with better prognoses and those with higher quality embryos and is not influenced by the day of transfer. Prior studies finding an increased risk with BL transfer were accurate but likely reflect an association reflecting the intrinsically better prognostic patients/embryos selected for extended culture. Clinicians and scientists should not hesitate to employ BL culture over concerns for MZT.