7KLVPDWHULDOPD\EHSURWHFWHGE\&RS\ULJKWODZ7LWOH86&RGH
`
`ORIGINAL RESEARCH ARTICLE
`
`Activity of the Pituitary-Ovarian Axis
`in the Pill-Free Interval During Use of
`Low-Dose Combined Oral
`Contraceptives
`
`A.M. van Heusden* and B.C.J.M. Fauser*
`
`This study was performed to evaluate pituitary-ovarian
`recovery in the pill-free interval during use of three low-
`dose combined oral contraceptives (COC). Either the es-
`trogen component or the progestin component was compa-
`rable in the study groups, to evaluate their relative
`influence. Serum luteinizing hormone (LH), follicle-stimu-
`lating hormone (FSH), and estradiol (E2) levels were mea-
`sured and follicle number and size estimated by transvag-
`inal sonography daily during the 7-day pill-free interval in
`44 healthy volunteers using three different low-dose oral
`contraceptives. Healthy volunteers were enrolled using 20
`␮g ethinyl estradiol (EE) ⫹ 75 ␮g gestodene (GSD) (Harmo-
`net威, Wyeth-Lederle; n ⫽ 15), 20 ␮g EE⫹ 150 ␮g deso-
`gestrel (DSG) (Mercilon威, Organon n ⫽ 17), or 30 ␮g EE⫹
`150 ␮g DSG (Marvelon威, Organon, n ⫽ 12) given according
`to the usual regimen of one tablet daily during 3 weeks and
`1 week pill-free interval.
`No ovulations were observed. Pituitary hormones were
`not statistically significantly different at the beginning of
`the pill-free interval between the study groups. FSH con-
`centrations were significantly higher at the end of the
`pill-free interval in the 30 ␮g EE group compared with both
`20 ␮g EE groups (7.0 [0.6–12.4] IU/L vs 4.9 [1.4–6.1] IU/L
`and 4.5 [2.4–7.4] IU/L; p ⫽ 0.001). In both 20 ␮g EE groups,
`a single persistent follicle (24 and 28 mm) was present in
`one subject. Follicle diameters were statistically signifi-
`cantly smaller at the beginning and at the end of the
`pill-free period in the 30 ␮g EE group compared with both
`20 ␮g EE study groups. Dominant follicles (defined as
`follicle diameter ⱖ10 mm) were observed at the end of the
`pill-free interval in both 20 ␮g EE groups (in 27% and 18%
`
`*Division of Reproductive Medicine, Department of Obstetrics & Gynecology,
`University Hospital Rotterdam & Erasmus University Medical School, Rotter-
`dam, The Netherlands
`Name and address for correspondence: A.M. van Heusden, Division of
`Reproductive Medicine, Department of Obstertrics & Gynecology, University
`Hospital Rotterdam & Erasmus University Medical School, 3051 GD Rotterdam,
`The Netherlands; Tel.: ⫹31-10-463-760; Fax: ⫹31-10-4367-306; e-mail:
`Fauser@gyna.azr.nl
`Submitted for publication November 30, 1998
`Revised March 24, 1999
`Accepted for publication March 24, 1999
`
`of women, respectively) but not in the 30 ␮g EE group.
`Finally, the area-under-the-curve for E2 was statistically
`significantly lower in the 30 ␮g EE group compared with
`both 20 ␮g EE groups.
`In conclusion, the EE content rather than the progestin
`component in the studied COC determined the extent of
`residual ovarian activity at the beginning of the pill-free
`interval. Dominant follicles were encountered only in the
`20 ␮g EE study groups. CONTRACEPTION 1999;59:237–243
`© 1999 Elsevier Science Inc. All rights reserved.
`
`KEY WORDS: ovarian activity, pill-free period, follicle devel-
`opment, oral contraception, follicle stimulating hormone,
`estradiol
`
`IntroductionThe contraceptive effect during the use of com-
`
`bined oral contraceptives (COC) is predomi-
`nantly established as a result of inhibition of
`the hypothalamic-pituitary-ovarian axis. Follicle
`growth is prevented and ovulation inhibited. The
`estrogen component is considered to inhibit follicle
`stimulating hormone (FSH) production and conse-
`quently to diminish FSH-dependent follicle growth.
`Should, however, a dominant follicle emerge, inhibi-
`tion of the luteinizing hormone (LH) surge and thus
`ovulation is prevented through the progestin compo-
`nent. The progestin component alone does not seem
`to have a prominent effect on basal concentrations of
`LH and FSH but, notably, inhibits peak concentrations.1
`Although not completely understood,
`inhibitory
`effects of both components in COC are established
`through synergistic interactions at the hypothalamic-
`pituitary level.2,3
`The 7-day pill-free period of most currently used
`regimens allows for withdrawal bleeding and serves
`the purpose of mimicking the normal menstrual
`cycle. In addition, it allows reduction of the overall
`amount of steroids administered over a 4-week pe-
`
`© 1999 Elsevier Science Inc. All rights reserved.
`655 Avenue of the Americas, New York, NY 10010
`
`Warner Chilcott v. Lupin Ltd., et al.,
`C.A. 11-05048 (JAP) (TJB)
`Warner Chilcott v. Watson Labs.,
`C.A. 12-2928 (JAP) (TJB)
`
`DTX 507
`
`ISSN 0010-7824/99/$20.00
`PII S00107824(99)00025-6
`
`Mylan v. Warner Chilcott IPR2015-00682
`WC Ex. 2007, Pg. 1
`
`

`
`238 van Heusden and Fauser
`
`Contraception
`1999;59:237–243
`
`riod. During the pill-free interval, pituitary-ovarian
`activity is allowed to resume in the absence of inhib-
`itory steroids until the next medication strip is initi-
`ated. Numerous publications have described recovery
`of ovarian activity during the pill-free interval or after
`pill omissions.4,5 However, daily blood sampling to-
`gether with ultrasound has rarely been performed.
`Few data are available to determine whether the
`magnitude of pituitary-ovarian suppression signifi-
`cantly differs among users of various low-dose COC.
`The present study compared resumption of pituitary-
`ovarian activity in women using three different low-
`dose COC to determine: 1) the maximum extent of
`suppression at the beginning of the pill-free interval,
`2) the magnitude of recovery of pituitary-ovarian
`activity during the pill-free interval, and 3) the extent
`of pituitary-ovarian activity at the end of the pill-free
`interval, a starting point for the next cycle.
`
`Materials and Methods
`Subjects and Study Protocol
`A total of 44 women using low-dose oral contracep-
`tion were included in this single-center group com-
`parative study in healthy female volunteers. The
`human ethics committee of the Dijkzigt Academic
`Hospital approved the study and all women gave
`written informed consent. The study was conducted
`according to the Declaration of Helsinki and the
`Good Clinical Practice (GCP) recommendations of
`the European Committee. Inclusion criteria were: age
`between 18 and 39 years; weight between 50 and 75
`kg; and cycle length between 24 and 35 days before
`starting the use of oral contraceptives. Excluded were
`women with hyperprolactinemia or polycystic ovary
`syndrome, contraindications for the use of oral con-
`traception, or any relevant medical disorder. Each
`volunteer entered the study on the first day of the
`pill-free period after the correct use of at least two
`cycles of study medication.
`Three low-dose COC were used for comparison:
`Fifteen women used 20 ␮g ethinyl estradiol (EE) ⫹ 75
`␮g gestodene (GSD)
`(Harmonet, Wyeth-Lederle,
`Hoofddorp, the Netherlands), 17 women used 20 ␮g
`EE ⫹ 150 ␮g desogestrel (DSG) (Mercilon, NV Or-
`ganon, Oss, the Netherlands) and 12 women used 30
`␮g EE⫹ 150 ␮g DSG (Marvelon, NV Organon, Oss,
`the Netherlands). Thirty-two women were randomly
`allocated to receive either 20 ␮g EE⫹ 150 ␮g DSG or
`20 ␮g EE⫹ 75 ␮g gestodene (GSD). Each subject was
`assessed in the pill-free period of the second cycle.
`The remaining 12 women using 30 ␮g EE⫹ 150 ␮g
`DSG were enrolled while using the study medication
`for at ⱖ2 months.
`
`Assessments
`All sonographic measurements were performed by a
`single investigator (A.M.vH.) using a 6.5 MHz trans-
`vaginal probe (Hitachi, Tokyo, Japan). Ovarian activ-
`ity was assessed by counting the number of follicles
`after scanning each ovary from the inner to the outer
`margin in a longitudinal cross-section, as previously
`described.6,7 The diameter was taken to be the mean
`of the size of the follicle in a longitudinal and an
`anteroposterior plane. Beyond a diameter of 10 mm,
`measurements in three planes were performed. Folli-
`cles ⱖ10 mm were considered dominant.6 – 8 On every
`occasion, endometrial thickness was assessed as the
`maximum thickness (both sides) present in the lon-
`gitudinal plane.
`Serum samples were centrifuged within 2 h after
`collection. Serum E2 levels were measured by radio-
`immunoassay (Diagnostics Products Corporation, Los
`Angeles, CA). Serum FSH and LH levels were deter-
`mined by immunoradiometric assay (Delfia kits, Kabi
`Pharmacia, Tu¨ rku¨ , Finland). Intra-assay and interas-
`say coefficients of variation (CV) were ⬍4.0% and
`⬍6.4% for FSH, ⬍15.5% and ⬍14.1% for LH, and
`⬍15% and ⬍18% for E2 , respectively. Samples from
`one individual were run in the same assay.
`
`Data Analysis
`Results are presented as median and range unless
`stated otherwise. Study parameters were compared
`using the Mann-Whitney U-test, a nonparametric test
`for comparison of two independent groups. Differ-
`ences were considered to reach statistical significance
`when p ⬍0.05.
`
`Results
`All 44 volunteers completed the study. Age, weight,
`and body mass index were not statistically signifi-
`cantly different among the study groups (data not
`shown). All women did use the studied COC for ⱖ2
`months before the assessments in this study and
`there were no pill omissions reported. Figures 1, 2,
`and 3 show the study parameters throughout the
`pill-free interval. Table 1 shows pituitary-ovarian
`activity at the beginning and end of the pill-free
`interval as well as parameters for the entire period.
`
`Pituitary activity
`In both 20 ␮g EE groups, a significant rise in serum
`FSH and E2 was observed from day 3 of the pill-free
`period onward (p ⬍0.01, Wilcoxon signed rank test).
`In the 30 ␮g EE group, this occurred from day 4
`onward.
`
`Mylan v. Warner Chilcott IPR2015-00682
`WC Ex. 2007, Pg. 2
`
`

`
`Contraception
`1999;59:237–243
`
`Ovarian Activity in the OC Pill-Free Period
`
`239
`
`Figure 1. Daily serum concentrations (IU/L) of LH (upper
`panel) and FSH (lower panel) during the pill-free period
`(days 1–7) in 44 healthy volunteers using three different
`combined oral contraceptive regimens. Data are presented
`as mean ⫾ SE.
`
`Figure 2. Daily maximum follicular diameters (mm) (up-
`per panel) and serum concentrations of E2 (pmol/L) (lower
`panel) during the pill-free period (days 1–7) in 44 healthy
`volunteers using three different combined oral contracep-
`tive regimens. Data are presented as mean ⫾ SE.
`
`20 ␮g EE⫹ 150 ␮g DSG versus 20 ␮g EE⫹ 75 ␮g
`GSD
`LH and FSH concentrations at the beginning (day 1) of
`the pill-free interval and at the end of the pill-free
`interval (day 7) were not significantly different. The
`area-under-the-curve (AUC) for both hormones was
`calculated as a measure for the total amount of
`hormone produced during this period. These were
`also not statistically significantly different.
`
`20 ␮g EE⫹ 150 ␮g DSG versus 30 ␮g EE⫹ 150 ␮g
`DSG
`Both LH and FSH concentrations were comparable on
`day 1. On day 7, FSH concentrations were statistically
`higher in 30 ␮g EE⫹ 150 ␮g DSG users (4.5 [2.4–7.4]
`IU/L versus 7.0 [0.6–12.4] IU/L; p ⫽ 0.001). Despite
`
`the different shape of the FSH concentrations in both
`groups, the AUC for FSH and LH was not different.
`
`Ovarian Activity
`No ovulation was observed in either group. In both 20
`␮g EE study groups, one volunteer with a persistent
`follicle was seen throughout the pill-free interval,
`ranging from 23.9 mm to 20.6 mm in the 20 ␮g EE⫹
`75 ␮g GSD group and 27.6 mm to 26.3 mm in the
`20 ␮g EE⫹ 150 ␮g DSG group. Because both struc-
`tures were accompanied by low E2 concentrations,
`they were omitted from comparative analysis and
`replaced by diameters from the second largest follicle
`present. A follicle ⱖ10 mm was present at the end of
`the pill-free interval in 4 of 15 women (27%) in the 20
`␮g EE⫹ 75 ␮g GSD group and in 3 of 17 women (18%)
`
`Mylan v. Warner Chilcott IPR2015-00682
`WC Ex. 2007, Pg. 3
`
`

`
`240 van Heusden and Fauser
`
`Contraception
`1999;59:237–243
`
`20 ␮g EE⫹ 75 ␮g GSD users, respectively. Finally,
`follicle diameters of the largest follicle present at the
`end of the pill-free interval were also not statistically
`different. E2 concentrations did not differ statistically
`significantly on day 1 or day 7 between the two
`groups. The AUC for E2 was also not different.
`
`20 ␮g EE⫹ 150 ␮g DSG versus 30 ␮g EE⫹ 150 ␮g
`DSG
`On day 1 of the pill-free interval, maximum follicle
`diameters were smaller in the 30␮g EE⫹ 150 ␮g DSG
`group, compared with the 20 ␮g EE⫹ 150 ␮g DSG
`group (p ⫽ 0.01). This difference was still present at
`the end of the pill-free interval (p ⫽ 0.02). Follicle
`growth was not different in the two groups: 2.9 mm
`(⫺0.2–7.0 mm) and 2.6 mm (⫺0.3–4.7 mm) for 30 ␮g
`EE ⫹ 150 ␮g DSG and 20 ␮g EE⫹ 150 ␮g DSG,
`respectively. E2 concentrations did not differ on days
`1 and 7 but the AUC for E2 was just significantly
`higher in the 20 ␮g EE group (p ⫽ 0.05).
`The correlation between FSH concentrations and
`LH concentrations on day 1 of the pill-free interval
`was consistent throughout different medication groups
`(Pearson’s r ⫽ 0.78, p ⫽ 0.0001). However, other
`correlations were less powerful: E2 and follicle diam-
`eter (Pearson’s r ⫽ 0.49, p ⫽ 0.001), FSH and E2
`(Pearson’s r ⫽ 0.34, p ⫽ 0.02), and E2 and LH (Pear-
`son’s r ⫽ 0.34, p ⫽ 0.02). At the end of the pill-free
`interval, the strongest correlation was found between
`follicle diameter and E2 concentration (Pearson’s r ⫽
`0.61, p ⫽ 0.001). Follicle growth correlated weakly
`with the AUC-FSH during the pill-free interval (Pear-
`son’s r ⫽ 0.32, p ⫽ 0.03).
`The presence of a dominant follicle at the end of the
`pill-free period was statistically significantly corre-
`lated with FSH on day 1 (p ⫽ 0.05), with E2 on day 1
`(p ⫽ 0.05), and with follicle diameter on day 1 (p ⫽
`0.01), but not with the study medication or EE dosage.
`
`Discussion
`During the luteal-follicular transition of the normal
`menstrual cycle, FSH levels surpass the threshold for
`stimulating ovarian activity. This intercycle rise in
`FSH elicits recruitment of a cohort of synchronous
`follicles from which a single dominant follicle is
`selected later in the follicular phase of the cycle.5
`Conventional combined oral contraceptives act pri-
`marily through inhibition of follicular growth in
`combination with peripheral progestin effects. In any
`situation in which medication is discontinued (either
`through “missing the pill” or a scheduled cessation of
`medication in the pill-free interval), recovery of pitu-
`itary-ovarian activity has been documented.9 –14 In
`the event of a scheduled 7-day pill-free interval,
`
`Figure 3. Daily serum concentrations of FSH (IU/L) (upper
`panel) and serum concentrations of E2 (pmol/L)
`(lower
`panel) during the pill-free period (days 1–7) in women who
`either have or do not have a dominant follicle present at the
`end of the pill-free period. Data are presented as mean ⫾ SE.
`
`in the 20 ␮g EE⫹ 150 ␮g DSG group, whereas in the
`30 ␮g EE⫹ 150 ␮g DSG group no follicles ⱖ10 mm
`were found. These differences were not statistically
`significant (Fisher’s exact test).
`
`20 ␮g EE⫹ 150 ␮g DSG versus 20 ␮g EE⫹ 75 ␮g
`GSD
`Maximum follicle diameters did not differ statisti-
`cally significantly at the start of the pill-free interval
`in the two 20 ␮g EE groups. Follicular growth of the
`largest follicle during the pill-free interval was similar
`in both groups: 2.9 mm (⫺3.0–7.1 mm) and 2.6 mm
`(⫺0.3–4.7 mm) for 20 ␮g EE⫹ 150 ␮g DSG users and
`
`Mylan v. Warner Chilcott IPR2015-00682
`WC Ex. 2007, Pg. 4
`
`

`
`Contraception
`1999;59:237–243
`
`Ovarian Activity in the OC Pill-Free Period
`
`241
`
`Table 1. Endocrine and ultrasound characteristics during the pill-free period in three low-dose, combined oral contracep-
`tives users
`
`Assessments
`
`LH
`(IU/I)
`
`FSH
`(IU/I)
`
`Max. fol.
`diameter
`(mm)
`E2
`(pmol/l)
`
`20 ␮g EEⴙ
`75 ␮g GSD
`Median/Range
`
`p*
`
`20 ␮g EEⴙ
`150 ␮g DSG
`Median/Range
`
`p†
`
`30 ␮g EEⴙ
`150 ␮g DSG
`Median/Range
`
`day 1
`day 7
`AUC 1–7
`day 1
`day 7
`AUC 1–7
`day 1
`day 7
`day 1–7†
`day 1
`day 7
`AUC 1–7
`
`0.7
`2.5
`12.5
`1.0
`4.9
`22.6
`5.6
`7.9
`2.9
`55
`161
`661
`
`0.05–6.2
`0.05–11.0
`1.2–41.8
`0.05–5.4
`1.4–6.1
`4.5–40.3
`3.9–10.7§
`5.4–14.4§
`⫺3.0–7.1
`12–97
`82–315
`343–1612
`
`0.2
`3.0
`10.3
`0.6
`4.5
`20.9
`5.2
`8.3
`2.6
`37
`167
`567
`
`0.05–3.7
`0.3–6.5
`3.9–34.6
`0.05–3.9
`2.4–7.4
`11.0–32.9
`4.0–15.2#
`5.5–14.9#
`⫺0.3–4.7
`14–90
`68–622
`293–2153
`
`0.001
`
`0.01
`0.02
`
`0.05
`
`0.14
`4.1
`12.9
`0.28
`7.0
`26.5
`4.5
`7.2
`2.9
`29
`116
`387
`
`0.05–4.6
`0.1–9.6
`0.4–46.4
`0.05–6.1
`0.6–12.4
`1.0–60.2
`3.5–6.3
`5.2–9.0
`⫺0.2–7.0
`20–52
`43–214
`295–907
`
`p‡
`
`0.02
`
`0.001
`
`0.05
`0.05
`
`0.02
`
`0.01
`
`Areas-under-the-curve (AUC) were calculated for endocrine assessments; the increase of the maximum follicular diameter is given for sonographic
`assessments
`Statistical differences were calculated between group 1 and 2 (p*), group 2 and 3 (p†), and group 1 and 3 (p‡).
`† follicular growth from day 1 until day 7
`§ In 1 patient, a persistent follicle was excluded (23.9 mm on day 1,20.6 mm on day 7).
`# In 1 patient, a persistent follicle was excluded (27.6 mm on day 1,26.3 mm on day 7).
`
`pituitary-ovarian activity should remain suppressed
`to an extent that the development of dominant folli-
`cles is prevented. These follicles carry the risk of
`ovulation or continued growth to become persisting
`follicles/cysts. The inhibitory effect of the contracep-
`tive combination should, therefore, be sufficient to
`arrest and repress the amount of activity present at
`the end of the 7-day pill-free interval. However, little
`is known about the influence of contraceptive com-
`binations or the relative importance of the estrogen
`and progestin component on the dynamics of pitu-
`itary-ovarian recovery during the pill-free interval.
`The present study focused on pituitary-ovarian activ-
`ity during the pill-free interval with three low-dose
`COC. By comparing Mercilon (20 ␮g EE⫹ 150 ␮g
`DSG) with either Marvelon (30 ␮g EE⫹ 150 ␮g DSG)
`or Harmonet (20 ␮g EE⫹ 75 ␮g GSD), the relative
`importance of the estrogen or progestin component
`was studied.
`At the start of the pill-free interval, 20 ␮g EE-
`containing COC are expected to have a lesser degree
`of pituitary suppression compared with the 30 ␮g EE
`COC due to a dose-dependent effect on FSH secre-
`tion.15 Although serum gonadotropin concentrations
`at the beginning of the pill-free interval were mark-
`edly reduced in the 30 ␮g EE study group, no statis-
`tically significant differences were found in gonado-
`tropin concentrations on the first day of the pill-free
`period between any of the study groups. This is
`probably largely due to interindividual variations in
`gonadotropin concentrations and the relatively small
`
`sample size. The rate of pituitary recovery in the 30
`␮g EE group appeared to be different compared to both
`20 ␮g EE groups; the increase of LH and FSH started
`more slowly but was more pronounced towards the
`end of the pill-free interval. FSH concentrations in the
`30 ␮g EE group were statistically higher compared
`with both 20 ␮g EE groups at the end of the pill-free
`period (see Table 1). It remains speculative whether
`the higher dosage of EE in the 30 ␮g EE group can be
`held responsible for the continued FSH suppression
`during the first days of the pill-free period. Reduced
`ovarian feedback by hormones produced by early
`antral follicles such as E2 and inhibin B may cause
`higher FSH levels at the end of the pill-free period in
`the 30 ␮g EE group.16
`Despite the lack of major differences in FSH and LH
`levels during the pill-free interval between the study
`groups, an important difference in ovarian activity
`was noted. In both study groups using 20 ␮g EE COC,
`a persistent follicle ⬎20 mm was observed at the
`beginning of the pill-free interval. Even when these
`were excluded from statistical comparison, maxi-
`mum follicle diameters were significantly smaller in
`the 30 ␮g EE group. The growth rate of the maximum
`follicle present (as measured by the increase in max-
`imum follicle diameter from day 1 to day 7) was not
`different between the study groups. No dominant
`follicles were seen in the 30 ␮g EE⫹ 150 ␮g DSG
`group, whereas 18%–27% of women in both 20 ␮g EE
`group presented with dominant follicles. In three of
`these six women, dominant follicles were already
`
`Mylan v. Warner Chilcott IPR2015-00682
`WC Ex. 2007, Pg. 5
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`

`
`242 van Heusden and Fauser
`
`Contraception
`1999;59:237–243
`
`present at the beginning of the pill-free period. The
`presence of a dominant follicle at the end of the
`pill-free period was statistically significantly corre-
`lated with FSH, follicle diameter, and E2 on the first
`day of the pill-free period. Also, FSH levels at the end
`of the pill-free interval in the 20 ␮g EE were lower.
`Figure 3 shows the different FSH and E2 profiles for
`women who have or do not have a dominant follicle
`at the end of the pill-free interval. It appears that the
`lower FSH levels at the end of the pill-free period
`might result from endogenous E2 feedback mecha-
`nisms if dominant follicles are present.
`Estradiol levels were not statistically significantly
`different either at the beginning or at the end of the
`pill-free interval among the study groups, although
`the AUC for E2 was statistically significantly higher
`in the 20 ␮g EE groups. Combined with follicular
`measurements, it appears that ovarian suppression is
`generally less during the pill-free interval in the 20 ␮g
`EE groups.
`Inasmuch as FSH concentrations and follicular
`growth rates during the pill-free interval do not differ
`between the 20 ␮g EE and 30 ␮g EE study groups, it is
`the follicular diameter at the beginning of the pill-free
`interval that probably provides the risk for selection
`and dominance. Once dominant follicles emerge, FSH
`levels will decline because of endogenous feedback
`mechanisms. This could explain the lower FSH levels
`in the 20 ␮g EE groups in this study. The more
`follicles mature during the pill-free period, the less
`FSH they need to continue development during the
`next cycle (ie, enhanced sensitivity for FSH of the
`maturing follicle). In other words, the more follicles
`are allowed to develop during the pill-free period, the
`more EE (and subsequent FSH suppression) is required
`thereafter. A suitable measure in that case would be a
`reduction of the length of the pill-free interval. Of
`course, well known interindividual differences exist
`in metabolism of contraceptive compounds17 and
`may indeed explain a reduction in control of pitu-
`itary-ovarian activity in many cases.
`In this study, modest differences in endocrine pa-
`rameters among the three studied populations were
`found. Interindividual differences within a group of-
`ten exceeded those between the groups. Although
`sample size may prevent the achievement of statisti-
`cal significance in some instances, the differences in
`follicle diameter may be clinically relevant. The
`present data suggest that a decrease in the EE content
`as seen in the 20 ␮g EE-containing COC results
`primarily in larger follicles during the pill-free inter-
`val. Because follicles maintain the potential to ovu-
`late,18 contraceptive efficacy in COC should include
`the prevention of dominant follicles.
`
`Acknowledgments
`We thank the staff of the Clinical Research Unit,
`Dijkzigt Hospital, for their logistic support.
`This study was financially supported by Stichting
`Voortplantingsgeneeskunde Rotterdam, Organon NV,
`and Wyeth-Ayerst.
`
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