FSH concentrations must exceed a certain level before follicular development will proceed.⁷ When this FSH threshold is surpassed in the normal cycle, the growth of a cohort of small antral follicles is stimulated and ensures ongoing preovulatory follicular development (Fig. 37-1).⁸ The duration of this period in which the threshold is exceeded (the FSH window) is limited in the normal cycle by a decrease in FSH, which occurs in the early to mid-follicular phase because of negative feedback from rising estrogen levels.

Figure 37-1 The intercycle rise in serum follicle-stimulating hormone (FSH) concentrations exceeds the threshold for recruitment of a cohort of follicles for further development. The number of follicles recruited is determined by the time (FSH window) for which the serum FSH is above the threshold at which recruitment occurs.

Adapted from Fauser BC, van Heusden AM: Manipulation of human ovarian function: Physiological concepts and clinical consequences. Endocr Rev 18:71, 1997.

Extending this window would allow the continuous development of more follicles. In the normal cycle, one follicle continues developing despite falling FSH levels because of an increased sensitivity to FSH ⁸ while the remaining follicles respond to falling FSH levels by undergoing atresia, as shown in Figure 37-1.

When the follicle reaches a certain level of development, sustained high circulating levels of estradiol produced by the preovulatory follicle will initiate the midcycle surge in luteinizing hormone (LH) that will trigger ovulation. The surge initiates a gene cascade in granulosa cells, the products of which initiate luteinization, signal the egg to commence meiotic maturation, and lead to rupture of the follicle wall.⁹

Corpus Luteum Development

The preovulatory surge of gonadotropins induces ovulation and differentiation of residual follicular cells that form the corpus luteum and begin to produce progesterone and estrogen at high rates. Theca cells express the enzymes necessary to convert cholesterol to androgens but lack the enzymes necessary to convert androgens to estrogens. Conversely, granulosa cells produce progesterone but are unable to convert pregnenolone or progesterone to androgens.

The preovulatory LH surge results in luteinization of granulosa and theca cells and alters the steroidogenic pathway so that progesterone is the primary steroid hormone produced after luteinization. However, the corpus luteum retains the ability to produce estrogen. Adequate luteal progesterone is secreted to allow maintenance of pregnancy until a conceptus-derived source of progesterone, the placenta, can produce adequate amounts of this steroid to support pregnancy.¹⁰

OVULATION INDUCTION

Indications

Normal follicular development culminates in ovulation of a mature oocyte, followed by the development of a corpus luteum producing adequate amounts of progesterone. This sequence of events is orchestrated by the interaction of local ovarian factors and endocrine factors from remote tissues, including the pituitary and hypothalamus. In addition, adequate functioning of other endocrine glands such as the thyroid and adrenal glands is important in orchestrating the interaction between the hypothalamus, pituitary, and ovaries that results in ovulation.

Any derangement of any of these systems may result in dysfunctional ovarian follicular development or even the complete failure of ovulation. The presence of subtle abnormalities despite the occurrence of ovulation is believed to be responsible, at least in part, for what is thought to be unexplained infertility in some women and perhaps for endometriosis-related infertility.

Ovulatory Dysfunction Classification

Clinically obvious anovulation or oligo-ovulation has been traditionally classified into three groups: World Health Organization (WHO) group I, II, and III anovulation.¹¹

Women in WHO group I suffer from a defect at the level of the hypothalamus/pituitary. They are estrogen deficient with normal or low FSH or prolactin levels and have no space-occupying lesion in the hypothalamic/pituitary region. They typically have amenorrhea and do not bleed in response to progestin challenge.

Women in WHO group II are not estrogen deficient. Their FSH and prolactin levels are normal. They typically experience oligomenorrhea, but they may have anovulatory cycles or amenorrhea with bleeding in response to a progestin challenge. Included in this group are women with polycystic ovary syndrome (PCOS).

Women in WHO group III include those with elevated gonadotropins secondary to primary ovarian failure mainly due to diminished ovarian reserve and the loss of ovarian follicles. These women are resistant to various methods of ovarian stimulation, and the best approach for their anovulation-associated infertility is oocyte donation.

Medications for Ovarian Follicle Stimulation

The available medications for stimulating ovarian follicular development include oral agents, mainly antiestrogens, better termed selective estrogen receptor modulators (SERM), and medications delivered by injections: gonadotropins and GnRH agonists. For more than four decades, an oral agent, clomiphene citrate, and injectable gonadotropins have almost been exclusively used for stimulating ovarian follicular development.

Gemzell and his coworkers announced the first successful induction of ovulation using human pituitary gonadotropins in 1958 and the first pregnancy in 1960.^12,¹³ One year later, Bettendorf and his group reported a similar experience.¹⁴ In 1961, Greenblatt and his coworkers published the first results of ovulation induction achieved by the use of clomiphene citrate (at that time known as MRL/41).¹⁵

In the past decade, insulin sensitizers have been introduced into clinical practice for ovulation induction for patients with PCOS and significant insulin resistance. Most recently, aromatase inhibitors have been used as an alternative to clomiphene citrate for ovulation induction or to improve the outcome of controlled ovarian stimulation with gonadotropins.

CLOMIPHENE CITRATE

For more than 40 years, clomiphene citrate has been the most commonly used oral agent for induction of ovulation. Since the early 1960s, results of clomiphene citrate treatment have not changed appreciably, despite the advent of modern immunoassays for steroid hormones, advances in ultrasound technology for cycle monitoring, and the introduction of commercial ovulation predictor kits that allow accurate identification of the midcycle LH surge.

It is interesting to note that clomiphene citrate is considered pregnancy risk category X. Studies in rats and mice have shown a dose-related increase in some types of malformations and an increase in mortality. This is particularly important when considering the relatively long half-life of about 5 days to 3 weeks (depending on the isomer) and that clomiphene citrate may be stored in body fat.¹⁶^–¹⁸ Fortunately, studies in humans have not found an association between clomiphene citrate and congenital defects.

Structure and Pharmacokinetics

Chemically, clomiphene citrate is a nonsteroidal triphenylethylene derivative and is chemically related to diethylstilbestrol and tamoxifen. Like the latter of these compounds, clomiphene can be considered to be a SERM that can exhibit both estrogen agonist and antagonist properties depending on the prevailing levels of endogenous estrogen. Estrogen agonist properties are manifest only when endogenous estrogen levels are extremely low. Otherwise, clomiphene citrate acts mainly as an antiestrogen.¹⁶

Clomiphene citrate is cleared through the liver and excreted in the stool. About 85% of an administered dose is eliminated after approximately 6 days, although traces may remain in the circulation for much longer.¹⁷ Clomiphene citrate is a racemic mixture of two distinct stereoisomers, enclomiphene and zuclomiphene, having different properties. Available evidence indicates that enclomiphene is the more potent antiestrogenic isomer and the one primarily responsible for the ovulation-inducing actions of clomiphene citrate.¹⁶^–¹⁸

Levels of enclomiphene rise rapidly after administration and fall to undetectable concentrations after few days. Zuclomiphene, the transisomer of clomiphene citrate, is cleared far more slowly; levels of this less active isomer remain detectable in the circulation for more than 1 month after treatment and may actually accumulate over consecutive cycles of treatment.¹⁸

Pharmacodynamics and Mode of Action

Clomiphene binds to estrogen receptors throughout the body due to its structural similarity to estrogen. However, unlike natural estrogen, which only binds estrogen receptors for a period of hours, clomiphene citrate binds estrogen receptors for an extended period of time, weeks rather than hours. Such extended binding ultimately depletes estrogen receptor concentrations by interfering with the normal process of estrogen receptor replenishment.¹⁵

The antiestrogenic effect of action of clomiphene citrate on the hypothalamus, and possibly the pituitary, is believed to be its main mechanism for ovarian stimulation. Depletion of hypothalamic estrogen receptors prevents correct interpretation of circulating estrogen levels; estrogen concentrations are falsely perceived as low, leading to reduced estrogen-negative feedback on GnRH production by the hypothalamus and gonadotropin (FSH and LH) production by the pituitary.

It is believed that the hypothalamus is the main site of action because in normally ovulatory women, clomiphene citrate treatment was found to increases GnRH pulse frequency.¹⁹ However, actions at the pituitary level may also be involved because clomiphene citrate treatment increased pulse amplitude, but not frequency in anovulatory women with PCOS, in whom the GnRH pulse frequency is already abnormally high.²⁰

During clomiphene citrate treatment, levels of both LH and FSH rise, then fall again after the typical 5-day course of therapy is completed. In successful treatment cycles, one or more dominant follicles emerge and mature, generating a rising tide of estrogen that ultimately triggers the midcycle LH surge and ovulation.^19,²⁰

Indications

The two main indications for clomiphene citrate treatment in women are (1) induction of ovulation in women with anovulatory infertility, and (2) stimulation of multifollicular ovulation or enhancing ovulation in ovulatory infertile women (e.g., unexplained infertility).

Clomiphene citrate is the initial treatment of choice for most anovulatory or oligo-ovulatory infertile women who are euthyroid and euprolactinemic while having adequate circulating levels of estrogen (WHO group II anovulation; e.g., PCOS). For anovulatory infertility, clomiphene citrate is effective only in conditions in which adequate levels of circulating estrogen exist to exert estrogen-negative feedback on gonadotropin production, and that could be antagonized by the antiestrogenic effect of clomiphene citrate. Women who do not respond to clomiphene citrate are those with very low circulating estrogen levels such as WHO groups I and III or women with defective hypothalamic-pituitary axis such as those with Sheehan’s syndrome and Kallmann syndrome.

For “unexplained” infertility, clomiphene citrate enhances the chance of achieving pregnancy apparently by stimulating multifollicular development as well as alleviating possible subtle ovulation dysfunction. The downside is an increased risk of multiple pregnancy, primarily twins.

Clomiphene Citrate Administration

Clomiphene citrate is administered orally for 5 days, starting on any day from the second to fifth day after the onset of spontaneous or progestin-induced menses. In anovulatory women, ovulation rates, conception rates, and pregnancy outcome are similar regardless of whether treatment is started on cycle day 2, 3, 4, or 5. Starting on day 5 has the theoretical advantage of giving more time to verify normal menses (and hopefully decreasing the chance of being pregnant) before starting therapy.

Treatment typically begins with a single 50-mg tablet daily for 5 consecutive days, increasing by 50-mg increments in subsequent cycles until ovulation is induced. Once the effective dose of clomiphene citrate is established, there is no indication for further increments unless the ovulatory response is lost. Higher doses will not improve the probability of conception, but the risk of hyperstimulation and multiple pregnancy might increase.

Although the dose required for achieving ovulation is correlated with body weight, there is no reliable way to predict accurately what dose will be required in an individual woman. Consequently, clomiphene citrate induction of ovulation amounts to an empirical incremental titration to establish the lowest effective dose for each individual.

The effective dose of clomiphene citrate for most women ranges from 50 to 250 mg/day, although some women will be found to be sensitive to clomiphene citrate and only require doses as low as 12.5 to 25 mg/day. Most women will ovulate at the lower doses, with 52% ovulating after 50 mg/day and another 22% after treatment with 100 mg/day. Although higher doses are sometimes required, the ovulation rates are usually low at these doses, with only 12% ovulating after 150 mg/day, 7% after 200 mg/day, and 5% after 250 mg/day. Most women who fail to respond to 150 mg/day of clomiphene citrate will ultimately require alternative or combination treatments.^21,²²

After the lowest ovulatory dose is determined, the same dose is repeated until pregnancy is achieved or a maximum number of approximately six cycles is reached. Pregnancy rates are highest during the first three cycles of clomiphene citrate treatment. The chances of achieving pregnancy are significantly lower beyond the third treatment cycle and uncommon beyond the sixth treatment cycle (Fig. 37-2). Therefore, it is rarely advisable to continue clomiphene citrate treatment beyond six treatment cycles.²¹

Figure 37-2 Cumulative pregnancy rates with clomiphene citrate and intrauterine insemination in patients with ovulatory and anovulatory infertility by Kaplan-Meier life table analysis. A, Filled circles, age <= 30; closed squares, age 31–35; open circles, age 36–40; open squares, age >= 41. B, Groups collapsed into two: closed circles, age <= 35; open squares, age >35.

From Agarwal SK, Buyalos RP: Clomiphene citrate with intrauterine insemination: Is it effective therapy above the age of 35 years? Fertil Steril 65:759–763, 1996.

The recommendation regarding which dose to apply is clear in cases of anovulatory infertility (increments of 50 mg until ovulation is achieved, then the dose is maintained). However, this is not clear in cases with ovulatory infertility. There are no adequate studies regarding which dose or what response (number of mature follicles) would be optimal in these women. This uncertainty is related to the individual variability in response to clomiphene citrate treatment irrespective of the dose and to the absence of a clear correlation between pregnancy rates and the number of mature follicles or amount of clomiphene citrate administered. Moreover, the value of clomiphene citrate treatment in enhancing the chance of achieving pregnancy in cases with ovulatory infertility has been questioned ²² and has not been proven by large controlled studies.

Treatment Monitoring

Monitoring is required to ensure that ovulation is occurring during a clomiphene citrate treatment cycle. For most patients treated with clomiphene citrate, ovulation monitoring can often be done with any one of a number of low-cost, highly convenient methods conducted in the setting of a general obstetrician and gynecologist practice. Objective evidence of ovulation and normal luteal function are keys to successful treatment. The choice may vary and should be tailored to meet the needs of the individual patient.²³

Basal Body Temperature

Basal body temperature (BBT) recordings provide a simple and inexpensive method for evaluating response to treatment. For this test, the woman takes and graphs her temperature every morning. A midcycle phase shift indicates that ovulation has occurred.

Urinary LH Detection Kit

An ovulation prediction kit can identify the midcycle LH surge in urine and more precisely defines both the interval of peak fertility (day of surge detection and the next 2 days).²⁴ Usually the LH surge is observed from 5 to 12 days after the last clomiphene citrate tablet.²⁵ Ovulation will generally follow within 14 to 26 hours of detection of an LH surge. Consequently, timed coitus or intrauterine insemination (IUI) on the day after the first positive urinary LH test will have the highest pregnancy rates.

Midluteal Progesterone

BBT charts and urinary LH detection kits can confirm that ovulation has occurred but do not document the quality of luteal function, which requires further tests—classically, a serum progesterone determination or endometrial biopsy.

A progesterone level above 3 ng/mL is evidence of ovulation.²⁶ Midluteal phase progesterone concentration (7 days after BBT shift or 7 to 9 days after urinary LH surge) offers more information. Levels exceeding 10 ng/mL are usually suggestive of an adequate luteal phase.²⁷ Endometrial biopsy and “secretory” histology that results from the action of progesterone also provides evidence of ovulation. Endometrial “dating,” using established histologic criteria, has been traditionally suggested for diagnosing luteal phase adequacy.²⁸ However, there is still much uncertainty about the importance of luteal phase defects diagnosed by serum progesterone levels or by endometrial biopsies, reflecting our poor state of knowledge regarding the actual mechanisms involved in endometrial receptivity and implantation.^29,³⁰

More sophisticated measures to monitor the outcome of clomiphene citrate treatment, including transvaginal follicular ultrasound scans and measurements of serum estradiol and LH levels, can usually be conducted in a subspecialist reproductive endocrinology practice. Because of the cost and logistic requirements involved, the method is generally reserved for patients in whom less complicated methods fail to provide the necessary information and for women who are at a high risk for developing serious complications such as severe ovarian hyperstimulation syndrome (OHSS) and multiple pregnancy.²³

In the past, it was generally recommended that patients receiving clomiphene citrate treatment be examined before starting each new treatment cycle to ensure that there was no significant residual ovarian enlargement or ovarian follicular cysts. However, it appears that small residual follicular cysts usually do not expand during continued treatment.³¹ Consequently, it is unnecessary to perform “baseline” ultrasound examinations before each new treatment cycle. However, it is reasonable to withhold treatment when symptoms suggest the presence of a large cyst or gross residual ovarian enlargement.²³

Outcome of Clomiphene Citrate Treatment

Clomiphene citrate treatment has been reported to successfully induce ovulation in 60% to 80% of properly selected women. More than 70% of those who ovulate respond at the 50-mg or 100-mg dosage level. In young anovulatory women in whom anovulation is the sole reason preventing them from conceiving, cumulative conception rates between 60% and 70% are observed after up to three successfully induced ovulatory cycles, and 70% to 85% after five cycles. Overall, cycle fecundity is approximately 15% in women who ovulate in response to treatment.²¹

In the reality of daily clinical practice, clomiphene citrate induction of ovulation usually results in much lower pregnancy rates, particularly in a subspecialty referral infertility practice. Important factors that adversely affect treatment outcome are increased age (particularly older than age 35), presence of other treated or untreated infertility factors, and increasing duration of infertility.³² Amenorrheic women are more likely to conceive than oligomenorrheic women, probably because those who already ovulate, albeit inconsistently (oligomenorrheic), are more likely to have other coexisting infertility factors. Generally speaking, failure to conceive within six clomiphene citrate-induced ovulatory cycles is a clear indication to expand the diagnostic evaluation to exclude other factors, change the overall treatment strategy, or both.²³

Adverse Effects and Risks of Clomiphene Citrate

Clomiphene citrate is generally well-tolerated. Some side effects are relatively common, but rarely are they persistent or severe enough to threaten completion of treatment. Although clomiphene citrate treatment does have intrinsic risks, they are typically modest and manageable. Adverse effects are generally divided into the side effects related to clomiphene citrate itself and the adverse effects related to induction of ovulation in general.

Side Effects

Hot flashes due to the antiestrogenic property of clomiphene citrate are dose-dependent and occur in approximately 10% of clomiphene citrate-treated women. Symptoms are transient, rarely severe, and typically resolve soon after treatment ends.

Visual disturbances, including blurred or double vision, scotomata, and light sensitivity, are generally uncommon (<2% prevalence) and reversible. However, there are isolated reports of persistent symptoms long after treatment is discontinued with more severe complications such as optic neuropathy. Whenever visual disturbances are identified, treatment should be stopped and alternative methods of ovulation induction should be considered.

Less specific side effects include breast tenderness, pelvic discomfort, and nausea, all observed in 2% to 5% of clomiphene citrate-treated women.³³ In addition, there are relatively common reports of premenstrual syndrome-type symptoms in women on clomiphene citrate.³⁴

Congenital Anomalies

There is no evidence that clomiphene citrate treatment increases the overall risk of birth defects or of any specific malformation. Several large series have examined the question and have drawn the same conclusion.^35,³⁶ Earlier suggestions that the incidence of neural tube defects might be higher in pregnancies conceived during clomiphene citrate treatment have not been confirmed by more recent studies.³⁷ A small study of pregnancy outcome in women inadvertently exposed to clomiphene citrate during the first trimester also found no increase in the prevalence of congenital anomalies.³⁸

Pregnancy Loss

More than one study has suggested that pregnancies resulting from clomiphene citrate treatment are at increased risk of spontaneous abortion compared to spontaneous pregnancies. A study reviewed outcomes of 1744 clomiphene pregnancies compared to outcomes of 3245 spontaneous pregnancies.³⁹ In this study, pregnancy loss was classified as either clinical if a sac was seen on ultrasound or if it occurred after 6 weeks’ gestation, or preclinical if a quantitative human chorionic gonadotropin (hCG) was greater than or equal to 25 IU/L and no sac was seen or abortion occurred earlier. In this study, when preclinical and clinical were considered together, the overall incidence of spontaneous abortion was slightly higher for clomiphene pregnancies than for spontaneous pregnancies (23.7% versus 20.4%). When considered separately, preclinical spontaneous abortions were also increased by clomiphene citrate overall (5.8% versus 3.9%) and for women at least 30 years old (8.0% versus 4.9%), but not for those younger than age 30 (3.7 versus 3.0%). In contrast, when clinical abortions were considered separately, clomiphene citrate did not increase the overall rate (18.0% versus 16.4%), but the rate was increased for women younger than age 30 (15.9% versus 11.2%).

More recent studies looking at rate of spontaneous miscarriage in 62,228 clinical pregnancies resulting from ART procedures initiated in 1996 to 1998 in U.S. clinics also found that spontaneous miscarriage risk was increased among women who used clomiphene citrate.^40,⁴¹ However, the results of these studies are not considered to be conclusive. The pregnancy loss seen in women after clomiphene treatment may actually be related to other comorbidities in these patients, such as insulin resistance, other genetic factors related to PCOS or unexplained infertility, endometriosis, and advancing maternal age.⁴²

Antiestrogenic Effects and Fertility

Clomiphene citrate treatment is associated with antiestrogenic effects that might have adverse effects on fertility. Because of the relatively long half-life of clomiphene citrate isomers, clomiphene citrate exerts undesirable and unavoidable antiestrogenic effects on the endocervix, endometrium, ovary, ovum, and embryo. Numerous studies in women and in various model systems have described adverse effects on the quality or quantity of cervical mucus, endometrial growth and maturation, follicular or corpus luteum steroidogenesis, ovum fertilization, and embryo development.

These adverse effects are believed to explain the “discrepancy” between the ovulation and conception rates observed in clomiphene citrate-treated patients. These effects appear to be more apparent at higher doses or after longer durations of treatment. It has been suggested that treatment with exogenous supplemental estrogen may help to minimize or negate these effects.^43,⁴⁴ However, there is little compelling evidence to support any beneficial effect of estrogen replacement to reverse these peripheral antiestrogenic effects.²³

Clomiphene citrate treatment has been shown to affect the quality and quantity of cervical mucus production as well as the structure of the endometrium.⁴⁵^–⁴⁷ However, cervical mucus score (based on quantity and quality) has not been found to be of proven prognostic value on the treatment outcome (i.e., achievement of pregnancy).

The endometrium is believed to be one of the most important targets of the antiestrogenic effect of clomiphene citrate and may explain a large part of the lower pregnancy rate and the possible higher miscarriage rate with clomiphene citrate. Successful implantation requires a receptive endometrium, with synchronous development of glands and stroma.⁴⁸

Studies have reported conflicting effects of clomiphene citrate on the endometrium 45–47,⁴⁹^–⁵¹ possibly due to different methodology used for endometrial assessment. However, a recent study has prospectively applied morphometric analysis of the endometrium, a quantitative and objective technique to study the effect of clomiphene citrate on the endometrium in a group of normal women. In this study, clomiphene citrate was found to have a deleterious effect on the endometrium, demonstrated by a reduction in glandular density and an increase in the number of vacuolated cells.⁵¹ In addition, a reduction in endometrial thickness below the level thought to be needed to sustain implantation was found in up to 30% of women receiving clomiphene citrate for ovulation induction or for unexplained infertility.⁴⁵ This observation has been confirmed by other studies.^46,⁴⁷

Decreased uterine blood flow during the early luteal phase and the peri-implantation stage has been suggested to explain, at least in part, the poor outcome of clomiphene citrate treatment.⁵¹ Other investigators have suggested the presence of other unrecognized factors associated with clomiphene citrate treatment failure.⁵²^–⁵⁴ Moreover, there is some evidence for a direct negative effect of clomiphene citrate on fertilization, and on early mouse and rabbit embryo development.^55,⁵⁶ In addition, there have been reports of low-quality oocytes associated with clomiphene citrate treatment. However, the negative effects of clomiphene citrate in some animal studies have not been confirmed by others,^53,⁵⁴ and at present it is unclear if there are adverse direct effects of clomiphene citrate on human oocytes and human embryo development.

Because of the above-mentioned evidence for adverse peripheral antiestrogenic effects of clomiphene citrate treatment, several investigators tried to reverse these effects by administering estrogen concomitantly with clomiphene citrate treatment. Some investigators have reported increased endometrial thickness and improved pregnancy rates with this approach ^57,⁵⁸; others have reported no benefit⁴⁴ or even a deleterious effect of estrogen administration.⁵⁹ Another approach has been to administer clomiphene citrate earlier during the menstrual cycle rather than starting on day 5,⁶⁰ in the hopes of allowing the antiestrogenic effect to wear off to some extent before ovulation and implantation. In view of the long half-life of clomiphene citrate isomers, this approach is unlikely to be successful. A third method has been to combine another SERM such as tamoxifen, which has more estrogen agonistic effect on the endometrium with clomiphene citrate or to use tamoxifen as an alternative to clomiphene citrate.⁶¹ However, none of these strategies have proved to be completely successful in avoiding the peripheral antiestrogenic effects of clomiphene citrate. A more recent publication has suggested that high-dose soy isoflavones may be able to overcome the antiestrogenic effect of clomiphene citrate on the endometrium.⁶² This report remains to be confirmed by other investigators.

To summarize, from the available evidence and accumulated clinical experience, it is difficult to conclude the exact significance of adverse antiestrogenic effects on pregnancy outcome in clomiphene citrate-treated women. It seems that individual variability exists regarding sensitivity to the peripheral antiestrogenic effects of clomiphene citrate, likely because of the complexity of estrogen receptor replenishment and activation, and individual differences in the pharmacokinetics of clomiphene citrate. The discrepancy in the success rates in achieving pregnancy between women with unexplained infertility and women with PCOS in response to clomiphene citrate treatment (higher rates in PCOS women) suggests that PCOS women may be less vulnerable to the antiestrogenic effects of clomiphene citrate on peripheral tissues. Recent findings of differences in the expression levels of receptors (androgen receptor, estrogen receptor α and β, progesterone receptor) and cofactors in human endometrium and myometrium in PCOS patients support this hypothesis.⁶³ For example, the p160 family of steroid receptor coactivators, known to facilitate the action of steroid receptors throughout the menstrual cycle, was found to be overexpressed in the endometrium of women with PCOS.⁸⁰ However, the presence of infertility factors, rather than an increased sensitivity to the antiestrogenic effects of clomiphene citrate, in non-PCOS women may explain the less favorable outcome of clomiphene citrate treatment.