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A new model for ovarian follicular development during the human menstrual cycle

      Abstract

      Objective

      To evaluate changes in ovarian follicle dynamics during the human menstrual cycle to test the hypothesis that folliculogenesis occurs in a wave-like fashion.

      Design

      Prospective longitudinal study.

      Setting

      Healthy volunteers in an academic research environment.

      Patient(s)

      Fifty healthy women of reproductive age (range 19–43 years) with a history of regular menstrual cycles not taking medications known to interfere with reproductive function were evaluated.

      Intervention(s)

      Transvaginal ultrasonography was performed daily for one interovulatory interval (IOI).

      Main outcome measure(s)

      Changes in the diameter and number of follicles ≥5 mm were evaluated.

      Result(s)

      Sixty-eight percent of women exhibited two waves of follicle development during the IOI and 32% exhibited three waves. Waves were characterized by an increase and subsequent decrease in the number of follicles ≥5 mm occurring in association with the growth of ≥2 follicles to ≥6 mm. A day effect and day by wave interaction were detected in the mean diameter of the largest three follicles and the number of follicles ≥5 mm.

      Conclusion(s)

      The follicular wave phenomenon in women provides a new model for ovarian function during the menstrual cycle and will improve our understanding of the ovarian response to fertility and hormonal contraceptive regimens.

      Keywords

      The term wave has been used ambiguously in the context of mammalian folliculogenesis and has led to confusion regarding our understanding of human ovarian function. Human follicular growth, in its entirety, begins at a diameter of approximately 0.03 mm (i.e., primordial follicles) and continues for more than 150 days (∼5 menstrual cycles) until ovulation occurs (
      • Gougeon A.
      Dynamics of follicular growth in the human a model from preliminary results.
      ). Histologic evaluation of human ovarian follicular development has been interpreted to mean that the recruitment of three to eleven small antral follicles (2–5 mm) occurs in the late luteal phase of the menstrual cycle (
      • Gougeon A.
      Qualitative changes in medium and large antral follicles in the human ovary during the menstrual cycle.
      ). A single follicle is then selected from this cohort in the early follicular phase to undergo continued growth and ovulation at midcycle (
      • Goodman A.L.
      • Hodgen G.D.
      The ovarian triad of the primate menstrual cycle.
      ,
      • Bomsel-Helmreich O.
      Ultrasound and the preovulatory human follicle.
      ). Limited follicular development has been thought to occur in the luteal phase due to the inhibitory effects of luteal P (
      • Gougeon A.
      • Lefevre B.
      Evolution of the diameters of the largest healthy and atretic follicles during the human menstrual cycle.
      ,
      • Gougeon A.
      Dynamics of human follicular growth a morphologic perspective.
      ,
      • Pellicer A.
      • Gaitin P.
      • Neuspiller F.
      • Ardiles G.
      • Albert C.
      • Remohi J.
      • et al.
      Ovarian follicular dynamics from basic science to clinical practice.
      ,
      • Mcgee E.
      • Hsueh A.
      Initial and cyclic recruitment of ovarian follicles.
      ).
      Gougeon (1986) described “waves” of ovarian follicular growth in women as the continuous entry of preantral resting follicles into the growing phase throughout the menstrual cycle (
      • Gougeon A.
      Dynamics of follicular growth in the human a model from preliminary results.
      ). Other investigators have also referred to waves of follicle development during the human menstrual cycle; however, no definitions of wave are given (
      • Baird D.T.
      • Baker T.G.
      • McNatty K.P.
      • Neal P.
      Relationship between the secretion of the corpus luteum and the length of the follicular phase of the ovarian cycle.
      ,
      • Dervain I.
      ,

      Gore M, Nayudu P, Vlaisavljevic B, Thomas N. Dominance is not what it seems: individual identification of ovarian follicles during the follicular phase. In program and abstracts of the 10th Annual Meeting of the ESHRE, Brussels. Oxford University Press, Oxford, UK

      ). Waves of folliculogenesis in animal models (e.g., bovine, equine, ovine) are defined as the synchronous growth of a group of follicles from which one or more follicles are selected for preferential growth (
      • Adams G.P.
      Comparative patterns of follicle development and selection in ruminants.
      ). The bovine estrous cycle, in particular, has been used as a model for the study of ovarian function in women (
      • Adams G.P.
      • Pierson R.A.
      Bovine model for study of ovarian follicular dynamics in humans.
      ). Follicular waves emerge in cows at regular intervals throughout the estrous cycle, and are preceded by an increase in FSH (
      • Adams G.P.
      • Matteri R.L.
      • Kastelic J.P.
      • Ko J.C.H.
      • Ginther O.J.
      Association between surges of follicle-stimulating hormone and the emergence of follicular waves in heifers.
      ). Two and three waves of follicular development are most commonly observed during the bovine estrous cycle (
      • Adams G.P.
      • Pierson R.A.
      Bovine model for study of ovarian follicular dynamics in humans.
      ,
      • Sirois J.
      • Fortune J.E.
      Ovarian follicular dynamics during the estrous cycle in heifers monitored by real-time ultrasonography.
      ,
      • Knopf L.
      • Kastelic J.P.
      • Schallenberger E.
      • Ginther O.J.
      Ovarian follicular dynamics in heifers test of two-wave hypothesis by ultrasonically monitoring individual follicles.
      ). The final wave of follicular development in the estrous cycle of cows is ovulatory, whereas all preceding waves are anovulatory (
      • Adams G.P.
      • Pierson R.A.
      Bovine model for study of ovarian follicular dynamics in humans.
      ,
      • Pierson R.A.
      • Ginther O.J.
      Follicular populations during the estrous cycle in heifers I. Influence of day.
      ). It has been speculated that the wave phenomenon of follicular development in animal species permits a species-specific number of follicles to continue to grow and have the potential to ovulate while minimizing attrition from the follicular reserve by suppressing recruitment between waves (
      • Adams G.P.
      Comparative patterns of follicle development and selection in ruminants.
      ).
      In contrast to a wave pattern of development, it has been suggested that a single follicle grows by chance during a hormonally privileged period of the menstrual cycle in women (
      • Gougeon A.
      Qualitative changes in medium and large antral follicles in the human ovary during the menstrual cycle.
      ). According to this theory, referred to as the Propitious Moment Theory (
      • Adams G.P.
      The state of the art lecture maximizing ovarian potential: comparative folliculogenesis.
      ), antral follicles grow and regress continuously until conditions are right for a gonadotropin surge. The gonadotropin surge then induces ovulation of the follicle that, by happenstance, was mature at exactly the right point in the cycle.
      High-resolution transvaginal ultrasonography is a very effective method of monitoring ovarian follicular growth in women (
      • Pierson R.A.
      • Adams G.P.
      Computer-assisted image analysis diagnostic ultrasonography and ovulation induction: strange bedfellows.
      ). Although this imaging tool does not allow observation of preantral and early antral (i.e., <2 mm) follicles, it provides precise visualization of follicles at advanced stages of antral development during the last 2 weeks of follicle growth (i.e., ≥2 mm). Clinical observations in women undergoing transvaginal ultrasonographic ovarian monitoring in our laboratory revealed substantial follicular development during the luteal phase of the menstrual cycle. These observations led to the notion that waves of ovarian follicular growth may occur in women, as documented in animal species. The objective of this study was to characterize the daily growth and regression of ovarian follicles in women during one interovulatory interval (IOI) to test the hypothesis that wave-like changes in the number and diameter of follicles during an IOI would be ultrasonographically detected.

      Materials and methods

      Sixty-three women were enrolled in the study. Participants were assessed, by history and physical examination, to be healthy women of reproductive age (mean age [±SD] 28.0 ± 6.9 years, range 19–43 years). Women who were pregnant or lactating within 6 months of enrolling in the study, had used hormonal contraception within 3 months of enrolling in the study, had a history of irregular menstrual cycles, were taking medications known or suspected to interfere with reproductive function, or were planning surgery during the study period were not eligible to participate. Information about ethnicity, smoking status, body mass index (BMI), gravidity, parity, and previous oral contraceptive use were obtained from each woman who took part in the study.
      Each participant underwent daily transvaginal ultrasonographic evaluation of her ovaries for one IOI. An IOI was defined as the interval from one ovulation to the subsequent ovulation. Scans were initiated 12 days after menses (i.e., before the first ovulation) and were continued until 3 days after the second ovulation. Ovulation was defined as the disappearance of a large follicle (≥15 mm) that had been identified by ultrasonography on the previous day and the subsequent visualization of a corpus luteum (CL) (
      • Hanna M.D.
      • Chizen D.R.
      • Pierson R.A.
      Characteristics of follicular evacuation during human ovulation.
      ).
      During each examination, follicles ≥2 mm were counted and measured. Follicles were first imaged in an approximately transverse plane. The image was frozen when the follicle appeared maximal and the longest and widest follicle dimensions were recorded. The transducer was then rotated 90 degrees and similar measurements were recorded. Follicle diameter was estimated as the average of the four measurements. High-resolution ATL Ultramark 9 HDI and HDI 5000 ultrasound machines with 5-9 MHz multifrequency convex array transducers (Advanced Technologies Laboratories, Bothell, WA) were used to acquire follicular data. Scans were performed by a single ultrasonographer (A.R.B.) approximately 90% of the time; a second ultrasonographer (R.A.P.) was available when the primary sonographer was not present. The study protocol was approved by the Institutional Review Board of the University of Saskatchewan and Saskatoon District Health.
      Two methods were used to characterize changes in follicle diameter during the IOI: the Identity Method, and the Non-Identity Method. The Identity Method (
      • Knopf L.
      • Kastelic J.P.
      • Schallenberger E.
      • Ginther O.J.
      Ovarian follicular dynamics in heifers test of two-wave hypothesis by ultrasonically monitoring individual follicles.
      ,
      • Pierson R.A.
      • Ginther O.J.
      Ultrasonic imaging of the ovaries and uterus in cattle.
      ) involved drawing sketches of all follicles ≥4 mm in each ovary immediately after each scan. The day-to-day identities of individual follicles were determined using the internal iliac blood vessels, the ovarian hilus, and the location of neighboring follicles and the CL within the ovary as landmarks. The diameter profiles of individual follicles that grew to ≥8 mm throughout the IOI were graphed for each woman. Only follicles ≥8 mm were identified because the extraordinary number of 4- to 7-mm follicles made it difficult to accurately determine the day-to-day identities of these small follicles. The Non-Identity Method (
      • Ginther O.J.
      A method for characterizing ultrasonically-derived follicular data in heifers.
      ) involved sorting all follicles ≥4 mm in descending order of diameter for each woman on every day during the IOI. The diameters of the follicles occupying the largest, second largest, third largest categories, and so on, were then plotted daily during the IOI, regardless of individual identity.
      The number of follicles ≥5 mm detected on each day of the IOI were graphed for each woman. Follicle number data were combined for both ovaries, based on the results of studies in animal models (
      • Pierson R.A.
      • Ginther O.J.
      Follicular populations during the estrous cycle in heifers II. Influence of right and left sides and intraovarian effect of the corpus luteum.
      ,
      • Ginther O.J.
      • Kastelic J.P.
      • Knopf L.
      Composition and characteristics of follicular waves during the bovine estrous cycle.
      ,
      • Ginther O.J.
      • Knopf L.
      • Kastelic J.P.
      Temporal associations among ovarian events in cattle during oestrous cycles with two and three follicular waves.
      ,
      • Ginther O.J.
      • Kastelic J.P.
      • Knopf L.
      Intraovarian relationships among dominant and subordinate follicles and the corpus luteum in heifers.
      ) and analyses of follicle number data that compared right side vs. left side ovulations and whether dominant follicles in the luteal phase grew ipsilateral or contralateral to the CL. Peak-to-peak and trough-to-trough intervals in the number of follicles ≥5 mm were evaluated for each woman during the IOI to determine whether follicles grew in a wave-like fashion. A trough was defined as a data point immediately preceded by at least two decreasing data points and immediately succeeded by at least two increasing data points. A peak was defined as the highest point between two troughs, preceded by an increasing trend and succeeded by a decreasing trend. When the peak or trough fell on either extreme of the x-axis, peaks succeeded by a decreasing trend and troughs preceded by an increasing trend were considered. If a peak or trough occurred during a plateau, the first data point of the plateau was used.
      An increase in the number of follicles ≥5 mm was defined as two successively increasing data points or one increasing data point followed by a plateau and an increasing trend thereafter. A decrease in the number of follicles ≥5 mm was defined as two successively decreasing data points or one decreasing data point followed by a plateau and a decreasing trend thereafter. When the increase or decrease originated or terminated in a plateau, the first data point in the plateau was used to demarcate the increase or decrease. An increase and subsequent decrease in the number of follicles ≥5 mm, occurring in association with the growth of at least two follicles to ≥6 mm, was considered a “wave” of follicular development.
      For statistical and illustrative purposes, follicle diameter and number data were normalized and centralized according to the number of follicle waves observed. Profiles of the mean diameter of the largest three follicles throughout the IOI, as determined by the Non-Identity Method, were normalized to the mean IOI for women exhibiting two or three waves of follicle development during the cycle. Profiles of the number of follicles ≥5 mm for each woman were normalized in the same manner. Wave emergence was defined as the day on which the largest follicle of the wave was detected at 4–5 mm. Diameter profiles of the largest follicle of each wave were centralized to the mean day of emergence of each wave. The number of follicles ≥5 mm were centralized to the mean day of emergence of the first wave and 7 days before the emergence of the second wave for women with two waves. Follicle number data were centralized to the mean day of emergence of the first wave, 6 days before the emergence of the second wave, and 3 days before the emergence of the third wave in women with three waves. Normalized and centralized follicle diameter and follicle number data were then truncated to day 27 (i.e., day 0 = first ovulation), and repeated measures analyses of variance were used to determine an effect of day, wave, or a day by wave interaction (PROC MIXED, SAS/STAT Software, Cary, NC).
      Fisher’s exact test was used to determine whether age, BMI, ethnicity, smoking, previous oral contraceptive use, gravidity, and parity influenced the number of waves exhibited during the IOI.

      Results

      Data from 13 of the 63 women enrolled in the study were excluded from analyses because of ovarian irregularities: 1 woman had an IOI >2 SD from the mean, 4 women exhibited luteal phases shorter than 2 SD from the mean, 1 woman had an ovarian dermoid cyst, and 7 women developed an anovulatory follicular cyst, hemorrhagic anovulatory follicle, or luteinized unruptured follicle during the study. The remaining 50 data sets were used to characterize follicular dynamics in the present study.
      Individual follicular diameter profiles were compared using the Identity and Non-Identity Methods (FIGURE 1, FIGURE 2). The Non-Identity Method was chosen to characterize diameter profiles because it provided information about follicles in the 4- to 7-mm range, which the Identity Method did not.
      Figure thumbnail GR1
      FIGURE 1Follicular diameter profiles for a woman who exhibited two follicular waves during the interovulatory interval as determined by the (A) Identity Method and (B) Non-Identity Method. Note that only follicles that grew to ≥8 mm could be individually identified from day-to-day using the Identity Method. Follicles that grew to ≥6 mm were identified using the Non-Identity Method. (A), Each follicle is represented by a different symbol. (B) ■ = largest follicle, □ = second largest follicle, and * = third largest follicle.
      Baerwald. Human ovarian follicular development. Fertil Steril 2003.
      Figure thumbnail GR2
      FIGURE 2Follicular diameter profiles for a woman who exhibited three follicular waves during the interovulatory interval as determined by the (A) Identity Method and (B) Non-Identity Method. Note that only follicles that grew to ≥8 mm could be individually identified from day-to-day using the Identity Method. Follicles that grew to ≥6 mm were identified using the Non-Identity Method. (A), Each follicle is represented by a different symbol. (B) ■ = largest follicle, □ = second largest follicle, and * = third largest follicle.
      Baerwald. Human ovarian follicular development. Fertil Steril 2003.
      No differences were detected between the proportions of right side vs. left side ovulations in women with two and three follicular waves (P>.05). Likewise, we did not detect any differences between the numbers or diameters of dominant follicles that grew ipsilateral or contralateral to the CL during the luteal phase (P>.05). Therefore, follicle number data were combined between ovaries.
      Nonrandom changes in the number of follicles ≥5 mm and the diameter of follicles ≥6 mm (Non-Identity Method) were observed in all 50 women during the IOI, indicating a wave pattern of follicle development. Thirty-four of the 50 women (68%) exhibited two waves of follicle development; the remaining 16 women (32%) exhibited three waves of follicle development. The final wave of the cycle was ovulatory and the preceding waves were anovulatory in all 50 women. None of the women evaluated exhibited only a single wave of follicle development during the IOI.
      A day effect (P<.0001) and a day by wave interaction (P<.0001) were detected in the mean diameter of the largest three follicles throughout the IOI as determined by the Non-Identity Method, indicating that the profiles for the mean diameter of the largest three follicles were different in women with two waves as compared with women with three waves.
      A day effect (P<.0001) and a wave effect (P=.02) were detected in the number of follicles ≥5 mm throughout the IOI. When follicle number data were centralized to the day of wave emergence, a significant day by wave interaction was detected (P=.01), indicating that the changes observed in the number of follicles ≥5 mm in women with two waves differed from the changes observed in women with three waves during the IOI. Superimposed follicle diameter and number data in women with two vs. three waves are illustrated in Figure 3 .
      Figure thumbnail GR3
      FIGURE 3Day-to-day profiles (mean ± SEM) of the number of follicles ≥5 mm detected (■) and the diameter of the largest follicle of each wave (Non-Identity Method, ○) for women exhibiting (A) two waves (n = 34) and (B) three waves (n = 16) during one interovulatory interval. Asterisks indicate follicle number data overlap.
      Baerwald. Human ovarian follicular development.Fertil Steril 2003.
      In evaluating the mean follicle number and diameter profiles in Figure 3A, two waves were visualized during the IOI. In Figure 3B, three waves in follicle diameter data were observed, whereas only two waves in follicle number data were apparent during the IOI. Individual follicle number profiles for women with three waves did exhibit three troughs followed by three peaks. However, the mean decrease in follicle number during wave 2 and the mean increase in follicle number during wave 3 overlapped making it appear that only two waves in follicle number were observed.
      The mean intervals between peaks (10.0 ± 0.4 days) and troughs (9.7 ± 0.6 days) were not different in women with three waves (P=.70). However, the peak-to-peak interval of 16.0 ± 0.7 days in women with two waves was longer than the trough-to-trough interval of 13.7 ± 0.5 days (P=.008).
      In women with two wave cycles, the IOI was shorter (P <.05) and the interwave interval was longer (P<.05) than in women with three wave cycles (Table 1 ). The day of emergence of the first follicular wave was similar between women with two and three waves, but the second wave emerged earlier (P<.05) in women with three waves as compared to women with two waves (Table 1).
      TABLE 1Characteristics (mean ± SEM; range) of ovarian follicular waves during the menstrual cycle in women.
      2 Waves (n = 34)3 Waves (n = 16)
      Wave 1Wave 2Wave 1Wave 2Wave 3
      Interovulatory interval (d)27.4 ± 0.4a (24–32)29.4 ± 0.6b (26–34)
      Interwave interval (d)14.7 ± 0.4a (10–20)11.9 ± 0.8b (6–19)6.6 ± 0.4c (5–11)
      Day of wave emergence−0.5 ± 0.3a (−3–3)14.2 ± 0.3c (11–18)−0.3 ± 0.5a (−5–3)11.6 ± 0.6b (6–15)18.2 ± 0.6d (14–23)
      Maximum follicle diameter (mm)9.0 ± 0.4a (5–16)21.7 ± 0.4b (17–26)9.2 ± 0.8a (6–17)9.7 ± 0.7a (6–15)20.7 ± 0.5c (17–25)
      a,b,c,d Within rows, values with no common superscript are different as determined by t tests, paired t tests, and analysis of variance with Scheffe’s post hoc tests (P<.05).
      Baerwald. Human ovarian follicular development. Fertil Steril 2003.
      Ovulatory follicles emerged in the early follicular phase and grew to a smaller preovulatory diameter in three wave cycles than in two wave cycles (P<.05; Table 1). Anovulatory follicular waves emerged 1 day before ovulation in women with two waves and on the day of ovulation and in the late luteal phase in women with three waves (Table 1). No differences were detected in the maximum diameter of the largest follicle of anovulatory waves in two wave and three wave cycles (P>.05; Table 1). The mean diameters of follicles from anovulatory waves were smaller than follicles from ovulatory waves in both two and three wave cycles (P<.05; Table 1). However, it is noteworthy that 4/66 (6%) anovulatory follicles grew to a preovulatory diameter of ≥15 mm and 20/66 (30%) grew to ≥10 mm.
      Age, BMI, ethnicity, smoking, previous oral contraceptive use, gravidity, or parity were not found to influence the number of waves detected during the IOI (P>.05).

      Discussion

      Our results supported the hypothesis that follicular development in women occurs in a wave-like fashion during the menstrual cycle. We observed nonrandom wave-like changes in follicle number and diameter and confirmed that women exhibit two or three waves of folliculogenesis during an IOI. This knowledge challenges the previously held notion that a single cohort of antral follicles grows only during the follicular phase of the menstrual cycle. The Identity and Non-Identity Methods of evaluating follicular dynamics in women during the IOI gave comparable results, similar to findings in animal models (
      • Ginther O.J.
      A method for characterizing ultrasonically-derived follicular data in heifers.
      ). The Non-Identity Method, however, was more useful for tracking the development of follicles <8 mm, because it did not require maintaining the day-to-day identities of the multitude of small (4–7 mm) follicles.
      Peak-to-peak and trough-to-trough intervals in follicle number for women with three follicular waves were not different, which supported a wave theory of follicle development. We did not observe similar peak-to-peak and trough-to-trough intervals in women with two follicular waves compared with women with three waves. We attributed this inconsistency in two wave cycles to error in determining the day of emergence of the first wave. The first wave emerged, on average, 1 day before ovulation in two wave cycles. Peaks in follicle number were often detected on the day of ovulation. Serial data were not available for all women on the days before the first ovulation. Therefore, the first peak in follicle number may have occurred earlier or later than we could detect accurately.
      A greater number of follicular waves during the cycle was associated with a longer IOI and shorter interwave interval. Only the final wave of follicle development was ovulatory, whereas all preceding waves were anovulatory. In both two and three wave cycles, the ovulatory wave emerged in the early follicular phase and anovulatory waves developed in the luteal phase. These observations are consistent with follicle wave dynamics as previously documented in animal models, particularly the bovine and equine models (
      • Adams G.P.
      Comparative patterns of follicle development and selection in ruminants.
      ,
      • Adams G.P.
      • Pierson R.A.
      Bovine model for study of ovarian follicular dynamics in humans.
      ,
      • Ginther O.J.
      Major and minor follicular waves during the equine estrous cycle.
      ).
      We postulate that the development of anovulatory follicles in the luteal phase occurs as a result of P-mediated inhibition of LH secretion to levels that allow follicular development to proceed to the antral or late antral stage, but do not allow the LH surge and ovulation to occur. Anovulatory follicles did not grow as large, on average, as ovulatory follicles. However, a notable number of women exhibited anovulatory follicles that grew to an ostensibly preovulatory diameter. It could, therefore, be speculated that follicles developing in the luteal phase of the cycle have the potential to ovulate in the presence of an LH surge.
      Previous studies in women have documented a greater incidence of right-side ovulations (
      • Fukuda M.
      • Fukuda K.
      • Andersen C.Y.
      • Byskov A.G.
      Right-sided ovulation favours pregnancy more than left-sided ovulation.
      ), while other studies have reported no difference (
      • Ecochard R.
      • Gougeon A.
      Side of ovulation and cycle characteristics in normally fertile women.
      ). However, we did not detect a difference in the number of right side vs. left side ovulations. It has also been reported that the CL exerted a negative effect on follicular growth in women (
      • Fukuda M.
      • Fukuda K.
      • Yding Andersen C.
      • Byskov A.G.
      Does corpus luteum locally affect follicular growth negatively?.
      ). The results of our study, and several other studies in animal models (
      • Pierson R.A.
      • Ginther O.J.
      Follicular populations during the estrous cycle in heifers II. Influence of right and left sides and intraovarian effect of the corpus luteum.
      ,
      • Ginther O.J.
      • Kastelic J.P.
      • Knopf L.
      Composition and characteristics of follicular waves during the bovine estrous cycle.
      ,
      • Ginther O.J.
      • Knopf L.
      • Kastelic J.P.
      Temporal associations among ovarian events in cattle during oestrous cycles with two and three follicular waves.
      ,
      • Ginther O.J.
      • Kastelic J.P.
      • Knopf L.
      Intraovarian relationships among dominant and subordinate follicles and the corpus luteum in heifers.
      ), however, do not support a negative effect of the CL on follicular growth. We did not detect differences in the numbers or diameters of dominant follicles that grew ipsilateral or contralateral to the CL during the luteal phase. We interpret these results to mean that the two ovaries act primarily as a single unit, and that ovarian follicular waves are regulated by systemic rather than local mechanisms as documented in the bovine model (
      • Ginther O.J.
      • Kastelic J.P.
      • Knopf L.
      Intraovarian relationships among dominant and subordinate follicles and the corpus luteum in heifers.
      ).
      We did not detect effects of age, BMI, ethnicity, smoking, previous oral contraceptive use, gravidity, and parity on the number of waves exhibited during the IOI. However, it is noteworthy that our failure to detect differences in the number of follicular waves in women of different ages and BMIs may have been due to small numbers of women ≤20 and >35 years of age and women with BMIs <20 and >35. Additional studies must be performed on women fitting broader demographic profiles before conclusions may be drawn.
      Further evaluation of these data are being performed in our laboratory to determine the role of the pituitary gonadotropins and ovarian steroid hormones in the development of ovarian follicular waves in women. These studies will help us to understand the mechanisms regulating the development of follicular waves during the menstrual cycle, as well as follicular development and ovulations observed during hormonal contraceptive use (
      • Pierson R.A.
      • Archer D.F.
      • Moreau M.
      • Audet M.C.
      • Fluker M.
      • Bouchard C.
      A contraceptive patch is significantly more effective than oral contraceptives (OCs) in suppressing follicular development.
      ,
      • Pierson R.A.
      • Archer D.F.
      • Moreau M.
      • Shangold G.
      • Fisher A.C.
      • Creasy G.W.
      Ortho Evra™/Evra™ versus oral contraceptives follicular development and ovulation in normal cycles and after an intentional dosing error.
      ,
      • Broome M.
      • Clayton J.
      • Fotherby K.
      Enlarged follicles in women using oral contraceptives.
      ,
      • Grimes D.
      • Godwin A.J.
      • Rubin A.
      • Smith J.A.
      • Lacarra M.
      Ovulation and follicular development associated with three low-dose oral contraceptives a randomized controlled trial.
      ,
      • Killick S.
      • Eyong E.
      • Elstein M.
      Ovarian follicular development in oral contraceptive cycles.
      ,
      • Hoogland J.H.
      • Skouby S.O.
      Ultrasound evaluation of ovarian activity under oral contraceptives.
      ).
      Documentation of a wave phenomenon of ovarian follicular development in women provides a new model for folliculogenesis during the human menstrual cycle. We anticipate that the knowledge of follicular waves during the menstrual cycle will have profound implications for infertility diagnoses and treatment in women. The development of more than one wave of follicular development during a woman’s cycle may provide women undergoing assisted reproduction with better opportunities for initiating ovarian stimulation protocols. This option would provide women with a more time-efficient and less expensive treatment regimen. Consideration of a wave model for ovarian follicular growth may also be useful for the development of more efficacious and user-friendly hormonal contraceptive regimens.

      Acknowledgements

      The authors thank the volunteers whose participation and dedication was invaluable for the completion of this study. Appreciation is expressed to John Deptuch for the expertise he provided in developing the computerized database for the storage and manipulation of our data.

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