Minimal Stimulation and Natural Cycle In Vitro Fertilization, 1st ed. 2015

8. Monofollicular Stimulation in PCOS Patients

Yanping Kuang Qingqing Hong  and Qiuju Chen 

(1)

Department of Assisted Reproduction, Shanghai Ninth People’s Hospital, Shanghai Jiaotong University School of Medicine, Zhizaoju Road 639, Huangpu District, Shanghai, 200011, People’s Republic of China

Yanping Kuang (Corresponding author)

Email: kuangyanp@126.com

Qingqing Hong

Email: qingqinghong@hotmail.com

Qiuju Chen

Email: chenqiuju2865@hotmail.com

Abstract

Polycystic ovary syndrome (PCOS) is the major cause of anovulatory infertility that affects up to 5–10 % of reproductive-age women. Very few studies have focused on monofollicular stimulation in vitro fertilization (IVF) in PCOS patients till now. The first-line medical ovulation induction therapy to improve fertility outcomes is Clomiphene citrate (CC); the recommended starting dose is 50 mg/day. Letrozole has been shown to have good ovulation rate in CC-resistant PCOS women; 5 mg/day showed an optimal result, with high monofolliculogenesis occurrence in PCOS women. When PCOS patients are undergoing IVF treatment, many of these women exhibit exaggerated response, resulting in an increased risk of ovarian hyperstimulation syndrome (OHSS) and multiple gestations. The transfer of frozen-thawed embryos has important implications for the management of women undergoing ovarian hyperstimulation for IVF. Frozen embryo transfer (FET) endometrial preparation by Letrozole combined with human menopausal gonadotropin (hMG) in PCOS is a novel protocol giving us optimal clinical results. The protocol of Letrozole combined with hMG has the potential to be the first-line treatment option for ovulation induction in PCOS women, while its use in ovarian stimulation for IVF deserves further study.

Keywords

PCOSClomiphene citrateLetrozoleGonadotropin therapyIVFFETEndometrial preparationLetrozole combined with hMG

Introduction

Polycystic ovary syndrome (PCOS) is the major cause of anovulatory infertility that affects up to 5–10 % of reproductive-age women. The first-line medical ovulation induction therapy to improve fertility outcomes is Clomiphene citrate followed by gonadotropins. There is currently insufficient evidence to recommend aromatase inhibitors over that of Clomiphene citrate in infertile anovulatory PCOS women in general or specifically in therapy-naive or Clomiphene citrate-resistant PCOS women. In vitro fertilization-embryo transfer (IVF-ET) is the last step treatment modality for child-desiring women with PCOS. These patients are potential high responders in controlled ovarian hyperstimulation (COH) cycles. High responders have a higher risk of OHSS, which is an iatrogenic complication of the COH protocol. Few studies have focused on monofollicular stimulation IVF in PCOS patients till now.

Diagnosis Criteria of PCOS

Polycystic ovary syndrome is a heterogeneous disorder of functional androgen excess, detectable either by laboratory analysis or by clinical examination, with ovulatory dysfunction and polycystic ovarian morphology, also affecting a large proportion of these patients. PCOS is a diagnosis of exclusion, with other androgen excess or related disorders to be ruled out. The first broadly used definition of PCOS arose from the proceedings of an expert conference sponsored by the NIH in 1990 (Zawadzki and Dunaif 1992), which noted the features of PCOS to be (in order of importance): (a) hyperandrogenism and/or hyperandrogenemia, (b) chronic anovulation, and (c) exclusion of related disorders such as hyperprolactinemia, thyroid disorders, and congenital adrenal hyperplasia. Another expert conference held in Rotterdam in 2003 (The Rotterdam ESHRE/ASRM 2004ab) expanded the NIH 1990 criteria for PCOS, noting that the disorder could be diagnosed by having two of the following three features: (a) oligo- or anovulation, (b) clinical and/or biochemical signs of hyperandrogenism, and (c) polycystic ovaries, after the exclusion of related disorders. This definition created two new phenotypes for PCOS: (a) women with polycystic ovaries and ovulatory dysfunction but no signs of androgen excess and (b) women with polycystic ovaries with clinical and/or biochemical evidence of androgen excess but no signs of ovulatory dysfunction.

To accommodate currently available data and to arrive at an evidence-based definition for PCOS, a modification of the NIH criteria was proposed by the AES in 2006 (Azziz et al. 2006). This definition recommended that PCOS be defined by three features: (a) androgen excess (clinical and/or biochemical hyperandrogenism), (b) ovarian dysfunction (oligo-anovulation and/or polycystic ovarian morphology), and (c) exclusion of other androgen excess or ovulatory disorders. Clearly, the prevalence of PCOS will depend on a degree on the criteria used to define this disorder.

Clomiphene

Clomiphene citrate (CC) is a long-standing standard drug for ovulation induction and is still considered as the first-line option in PCOS women, but there are some drawbacks with the use of it. Clomiphene citrate is a selective estrogen receptor modulator with a long half-life (2 weeks) and long-lasting estrogen receptor (ER) depletion (Young et al. 1999; Mikkelson et al. 1986) properties. Prolonged ER depletion may have an adverse effect on the cervical mucus and endometrium in 15–50 % of patients (Gonen and Casoer 1990; Sereepapong et al. 2000) causing a discrepancy between ovulation and conception rates and higher incidence of miscarriage (Homburg 2005; Al-Fozan et al. 2004; Franks et al. 1985; Kistner 1965). In addition, prolonged estrogen receptor downregulation can lead to multiple follicular growth and increased risk of multiple pregnancies and ovarian hyperstimulation syndrome (OHSS) which is of great concern in PCOS patients (Mitwally et al. 2005). Also, Clomiphene resistance occurs in 15–20 % of patients and is not predictable before beginning the treatment (Franks et al. 1985). Clomiphene is given for 5 days following the onset of a spontaneous or a progestagen-induced period, starting any time from days 2, 3, 4 and 5, as there is no difference in the outcome between these time points (Wu and Winkel 1989). The recommended starting dose is 50 mg/day, as almost half of the pregnancies are achieved with this dose (Gysler et al. 1982). A simplified monitoring, when treatment starts on day 2, involves the measurement of serum progesterone values on days 21 and 28 of the cycle (Messinis and Milingos 1997). Unless normal ovulation occurs (progesterone ≥30 nmol/l), the dose is increased in each of the next cycles by 50 mg/day up to a maximum dose of 150 mg/day. As yet, no studies have investigated whether more intensive monitoring, such as by ultrasound, is needed during ovulation induction with Clomiphene, unless intrauterine insemination (IUI) is also applied. Clomiphene induces ovulation at a high rate (70–90 %), and, although the pregnancy rate is relatively lower (30–40 %) (Messinis 2002), in properly selected patients with no other causes of infertility, it can be as high as 60 % after six cycles (Messinis and Milingos 1997) and 97 % after ten cycles (Hammond et al. 1983). The reasons for the relatively low pregnancy rate are not clear but may be related to the high luteinizing hormone (LH) levels, the anti-estrogenic effects of Clomiphene, and adverse effects on the oocytes (Wramsby et al. 1987; Homburg et al. 1988). It should also be noted that the earlier studies did not use uniform criteria for patient classification. Although high in earlier studies, the miscarriage rate in the most recent studies is similar to that in the normal population (Hammond et al. 1983; Messinis and Milingos 1997). The multiple pregnancy rate is 6–8 %, mainly twins (Adashi 1996), which is a rather low rate as compared to classical gonadotropin regimens (Wang and Gemzell 1980) but similar to that in the low-dose follicle-stimulating hormone (FSH) protocols (Franks and White 2002). OHSS is a rare event (Adashi 1996).

Letrozole

Letrozole (4,4′-[1H-1,2,4-triazol-1-ylmethylene]-bis-benzonitrile) is the third-generation aromatase inhibitor. It is an orally active aromatase inhibitor, with good potential for ovulation induction. It has been in use for few years now, and a number of researchers have studied this molecule as an option for ovulation induction. Administering aromatase inhibitors early in the follicular phase can induce ovulation by releasing the hypothalamus or pituitary from estrogen (E) negative feedback on gonadotropin-releasing hormone (GnRH) and gonadotropin secretion, which would stimulate ovarian follicular development. An alternative hypothesis is that aromatase inhibitors may act locally in the ovary to increase follicular sensitivity to FSH by accumulation of intraovarian androgens (Vendola et al. 1999). In addition, androgen accumulation in the follicle may stimulate insulin-like growth factor I (IGF-I), along with other endocrine and paracrine factors, which may synergize with FSH to promote folliculogenesis (Weil et al. 1999). Due to its short half-life (45 h) and the lack of downregulation of estrogen receptors (Casper 2003), it may have less negative effects on the endometrium and cervix in the late follicular phase (Casper and Mitwally 2006; Baruah et al. 2009). Mitwally and Casper (2001) showed that Letrozole, as an aromatase inhibitor, has minimal effect on the endometrium and compared with CC, Letrozole is associated with a thicker endometrium. Because Letrozole does not deplete estrogen receptors, normal central feedback mechanisms remain intact and mono-ovulation should occur in a 5-day course of treatment (Casper and Mitwally 2006). This is an advantage in PCOS because theoretically, it can reduce the risk of multiple pregnancy and OHSS (Casper and Mitwally 2006; Badawy et al. 2009).

There are several reports confirming the efficacy and superiority of Letrozole to Clomiphene (Baruah et al. 2009; Begum et al. 2009; Sohrabvand et al. 2006; Atay et al. 2006; Ganesh et al. 2009) or at least equal (Badawy et al. 2009; Bayar et al. 2006a; Zeinalzadeh et al. 2010; Abu Hashim et al. 2010) to Clomiphene in ovulation and pregnancy rates. The ideal daily dose of Letrozole, however, remains unconfirmed. Mitwally and Casper (2001) described the use of 2.5 mg Letrozole on days 3–7 of menses in women with PCOS. Ovulation occurred in nine patients (75 %) and pregnancy occurred in three (25 %). In a clinical randomized trial comparing different doses of Letrozole to induce ovulation in patients with unexplained infertility, higher doses of Letrozole (5 and 7.5 mg), although having the potential advantage of requiring a shorter period of stimulation, have been found to offer no significant advantage in comparison with the use of 2.5 mg in terms of pregnancy rates (PRs) (Badawy et al. 2007). Different doses of Letrozole, ranging from 2.5 to 7.5 mg/day, have been used in these studies; 5 mg/day Letrozole in PCOS patients showed an optimal result (Yang et al. 2008; Al-Fadhli et al. 2006; Badawy et al. 2007; Ramezanzadeh et al. 2011).

Letrozole has been shown to be effective in ovulation induction with a good ovulation rate in CC-resistant PCOS women (Ganesh et al. 2009). Indian PCOS women have high prevalence of insulin resistance (Pandit et al. 2012) and thus, are likely to have high CC resistance. Letrozole could prove to be a good alternative for ovulation induction in such women. Hyperinsulinemia, which is closely associated with PCOS, is thought to be one of the causative factors for CC resistance. The prevalence of insulin resistance in PCOS is close to 50 % (Glintborg et al. 2004). This could be one more reason for Letrozole to be a better first-line drug compared to Clomiphene citrate. Others reported similar results (CC 70.9 %, Let 67.5 %; Badawy et al. (2009)) (CC 74.7 %, Let 65.7 %; Bayer et al. (2006b), and (CC 72 %, Let 86 %; Zeinalzadeh et al. (2010)). In majority of the studies, no statistically significant difference was found between CC and Letrozole in the ovulation rate. Letrozole resulted in monofolliculogenesis in 79.49 % of cases, which is optimal for ovulation induction in PCOS women. However, where multiple follicular development is needed, Letrozole may be inadequate.

Gonadotropin Therapy

Ovulation induction with gonadotropins began in the 1960s, and there is a large body of observational evidence supporting the use of gonadotropin ovulation induction in Clomiphene citrate resistance (CCR) or Clomiphene citrate failure (CCF) PCOS women (PCOS Australian Alliance 2011). The low-dose step-up regimen employs a starting daily dose of 37.5–50 IU/day, which is only increased after 14 days if there is no response and then only by half an ampule every 7 days (Hamilton-Fairley et al. 1991). An established method for patients with PCOS is the “low-dose step-up” protocol, which involves a starting FSH dose of 75 IU/day given for 7–14 days (Polson et al. 1987). Treatment starts any time, provided low ovarian activity is present and is monitored by ultrasound scans. Unless a follicle ≥12 mm is seen in the ovaries, the dose is increased by 37.5 IU/day at weekly intervals up to a maximum dose of 225 IU/day. Human chorionic gonadotropn (hCG) is injected when the leading follicle is ≥18 mm in diameter with no other follicles >14 mm, although in these patients, the positive feedback mechanism is intact (Messinis and Milingos 1997). Treatment cycles using this approach can be quite long (up to 28–35 days) but the risk of multiple follicular growth is lower than with conventional step-up regimens. The initiation of follicular growth requires a 10–30 % increment in the dose of exogenous FSH and the threshold changes with follicular growth, due to an increased number of FSH receptors, so that the concentration of FSH required to maintain growth is lesser than that required to initiate it. In ovulation induction protocols, stimulation with gonadotropins does not require a background of pituitary desensitization.

Another approach to the treatment of PCOS patients with gonadotropins is the “step-down” protocol. The most recent modification of this protocol involves the administration of FSH at a starting dose of 150 IU/day until a follicle ≥10 mm is seen by ultrasound (Macklon and Fauser 2002). The dose is then decreased by 37.5 IU/day and further to 75 IU/day 3 days later and is kept constant until the day of hCG administration. Monofollicular development has been found in 56 % of the cycles with a pregnancy rate of 16 % per treated cycle and a cumulative pregnancy rate of 47 % (van Santbrink et al. 1995). These results have been considered comparable to those obtained with the step-up protocol, but with the step-down approach, a shorter duration of treatment and a smaller total dose of FSH have been reported (van Santbrink et al. 1995). Such conclusions, however, are based on a comparison between data from different studies (Macklon and Fauser 2002) and not between groups in the same study.

To date, there is no difference in efficacy between the different gonadotropin preparations (Nugent et al. 2000). It can be very challenging to stimulate the development of a single dominant follicle in women with PCOS. A systematic review of 14 randomized controlled trials (RCTs) (Nugent et al. 2000) found no significant differences between human menopausal gonadotropin (hMG) and urinary FSH (uFSH) in terms of pregnancy rate per cycle, multiple pregnancy rate, miscarriage rate, ovulation rate per cycle, or overstimulation rate per cycle. The exact risk of OHSS with gonadotropins is difficult to establish as it depends on the protocol used with higher rates when the step-down regimen is used. In order to reduce the risk of multiple pregnancy and OHSS, careful ultrasound monitoring with low-dose step-up regimens with starting doses of 25–50 IU, aiming for unifollicular development is employed. Two recently published RCTs (Lopez et al. 2004; Homburg et al. 2012) have suggested a possible higher efficacy with gonadotropins for ovulation induction compared to Clomiphene citrate in therapy-naive PCOS women, although the differences in cost and convenience may limit the choice of FSH as the first-line treatment.

IVF/ICSI Treatment in PCOS

IVF/ICSI treatment is not indicated for anovulation alone in the woman with PCOS but is recommended either as a third-line treatment (after failed first- or second-line therapies, including Clomiphene citrate, gonadotropin, or laparoscopic ovarian drilling or in the presence of other infertility factors such as tubal damage, severe endometriosis, and male factor infertility (Thessaloniki ESHRE/ASRM 2008). Women with PCOS undergoing IVF treatment have similar pregnancy, miscarriage, and live birth rates compared with those of non-PCOS patients, as evidenced by a large systematic review and meta-analysis of nine observational studies comparing 458 PCOS women (793 cycles) with 694 matched controls (1116 cycles) (Heijnen et al. 2006). Ovarian stimulation in women with PCOS poses a particular challenge, as many of these women exhibit exaggerated response, resulting in an increased risk of ovarian hyperstimulation syndrome (OHSS) and multiple gestations (MacDougall et al. 1993).

The idea behind the use of GnRH agonists in patients with PCOS was to suppress basal LH values when elevated and, therefore, to alleviate any adverse effects that high tonic LH might have on the outcome of treatment. Although earlier data regarding ovulation and pregnancy rates using the GnRH agonists in FSH-treated cycles were encouraging (Fleming et al. 1985; Dodson et al. 1987), subsequent studies demonstrated an increased risk of OHSS (Homburg et al. 1990; Scheele et al. 1993; van der Meer et al. 1996). This was evident even when a starting dose of FSH as low as 37.5 IU/day was used (Buckler et al. 1993). A retrospective analysis of data has shown that during treatment with FSH, the use of a GnRH agonist leads to a significant reduction in the miscarriage rate (Homburg et al. 1993), but this has not been confirmed prospectively (Clifford et al. 1996). For these reasons and the fact that basal LH can also decline during treatment with FSH alone (Kamrava et al. 1982; Sagle et al. 1991; Messinis and Milingos 1997), GnRH agonists are not recommended as a treatment of choice for ovulation induction in PCOS. The increased incidence of OHSS is attributed to the low percentage of monofollicular development with the use of GnRH agonists, which in one study was found to be as low as 22 % as compared to 80 % with low-dose FSH alone (van der Meer et al. 1996).

Monofollicular Stimulation for FET Endometrial Preparation in PCOS

The transfer of frozen-thawed embryos has important implications for the management of women undergoing ovarian hyperstimulation for IVF. By providing the possibility of a further embryo transfer, this strategy increases the cumulative pregnancy rate and reduces cost (Lieberman et al. 1992). Various attempts have been made to improve the success of frozen-thawed embryo transfer (FET) (Cohen et al. 1986), since Trounson and Mohr (1983) reported the first successful pregnancy in 1983. However, natural cycles or hormone replacement cycles similar to natural cycles have been used mostly in FET cycles (Ghobara and Vandekerckhove 2008).

Cryopreservation is nowadays an essential part of cost-effective ART programs and sound evidence has proved the safety of FET in comparison with fresh embryo transfer. FETs can be performed instead of fresh ETs to improve the outcome of ART not only for OHSS-risk patients but also for normal responders. Clinical data suggest that cryopreservation of all embryos by vitrification and transfer in a subsequent cycle may be an effective strategy to enhance outcome of ART. 374 patients enrolled were randomly divided into FET and fresh embryo transfer group, each group of 187 cases; the ongoing pregnancy rate in FET group was 39 %(n = 73), and fresh embryo transfer was 27.8 % (n = 52). It is suggested that fresh embryo transfer could be replaced instead by FET (Aflatoonian et al. 2010a). 103 patients with blastocyst transfer, enrolled by Shapiro et al. (2011), were randomly divided into two groups: 53 fresh embryo transfers and 50 FET; the clinical PR in the FET group was 84 %, implantation rate, 70.8 %, and ongoing PR (gestation 10 week), 78 %, whereas in the fresh embryo transfer group these values were 54.7 %, 38.9 %, 50.9 %, respectively. The neonatal outcome of 200 FET and 500 fresh embryo transfers was compared by Aflatoonian et al.; there was no significant difference between the two groups (Aflatoonian et al. 2010b). The neonatal outcome of 2293 FET, 4151 fresh embryo transfers, and 31,936 spontaneous pregnancies in 1995–2006 was compared; there was no significant difference between FET and fresh embryo transfer group in premature delivery rate, low birth weight, and intrauterine growth retardation (IUGR) (Pelkonen et al. 2010). The safety of vitrification of embryos at the six- to eight-cell stage is confirmed (Desai et al. 2010). A Japanese group followed 413 FET cycles over 4 years; 967 vitrified blastocysts were transferred, 147 babies were delivered, and the birth defects rate was 1.4 % (Takahashi et al. 2005).

Wikland et al. compared the neonatal outcomes of 106 vitrified blastocysts, 207 fresh blastocysts, and 206 slow freezing blastocysts in 2006–2008; the safety of vitrification was confirmed (Wikland et al. 2010). An Indian group observed 285 FET cycles; 817 vitrified D3 embryos were transferred; clinical pregnancy rate, implantation rate, abortion rate, and live birth rate were 36.84 %, 18.11 %, 7.71 %, and 24.21 %, respectively; 89 babies were delivered; and their birth defect rate was 1.18 % (Rama Raju et al. 2009).

Our center (Department of Assisted Reproduction, the Ninth People’s Hospital affiliated to Medical College of Shanghai Jiao Tong University, Shanghai, China) attempted to replace fresh ET by the strategy of freezing all embryos and FET for normal or high responders since March 2011. The clinical pregnancy rate of FET cycles was 40.7–51.5 % since then. The advantages of the separation of induced ovulation and embryo transfer are obvious: minimizing the proportion of pharmacological and surgical treatments, suitable protocols (e.g., CC mild stimulation, luteal phase ovulation induction) according to ovarian response, and lowering the risk of OHSS and multiple pregnancies, thereby increasing the safety for the mother and child.

FET endometrial preparation by following stimulation with Letrozole in PCOS as a novel protocol, is generally used in our center. Oral Letrozole, 5 mg per day for 5 days, was given from day 3 of menses. hMG (150 IU) was given every other day if leading follicle <14 mm. In patients whose leading follicles (≥14 mm) number was less than 3, ovulation was induced by hCG 10,000 IU. If women had more than 3 dominant follicles, GnRH agonist, 0.1 mg was administered. If LH was ≥ 20mIU/mL on the trigger day, 4/6 days later, thawed D3 embryos/blastocysts were transferred. When LH was <20mIU/ml on the trigger day, 5/7 days later, thawed D3 embryos/blastocysts were transferred (Fig. 8.1 ). All patients started oral Dydrogesterone, 40 mg/d for 16–18 days, from the day after ovulation. Two weeks after the FET procedure, an hCG assay was performed. If the assay was positive, transvaginal ultrasonography was scheduled for 2 weeks later. The pregnant woman continued Dydrogesterone till 10 weeks gestation.

A312222_1_En_8_Fig1_HTML.gif

Fig. 8.1

Method of endometrial preparation by Letrozole in PCOS

Five hundred and one PCOS patients were enrolled and 501 FET cycles performed between 2011 and 2012. The clinical pregnancy rate was 51.1 % (256/501), implantation rate was 36.8 % (349/948), miscarriage rate was 18 % (46/256), and live birth rate was 41.9 % (210/501). 67.1 % (336/501) cycles developed only one leading follicle (≥14 mm), two follicles ≥14 mm developed in 19.6 % (98/501) cycles, and three dominant follicles appeared in 6.8 % (34/501) cycles. In 6.6 % (33/501) cycles, there were more than three follicle ≥14 mm. Average serum estradiol level was 333 ± 240 pg/mL on the trigger day. Endometrial thickness was 12.1 ± 2.9 mm on ET day. There was no OHSS case in our study. The clinical outcome according to the leading follicle number is demonstrated in Table 8.1. Estradiol levels were significantly high in the group with more than three leading follicles. There was no significant difference in the clinical pregnancy rate and implantation rate in the four groups. It is suggested that Letrozole is an attractive option with its oral route of administration, cost, safety profile, and effectiveness in ovulation induction and ovarian stimulation. The protocol of Letrozole combined with hMG has the potential to be the first-line treatment option for ovulation induction in PCOS women, while its use in ovarian stimulation for IVF deserves further study.

Table 8.1

Data of different groups according to leading follicle number

Follicle number

ET day

Trigger day

Clinical pregnancy rate (%)

Implantation rate

ENT

E2

P

E2

P

1

12.1 ± 2.8

80 ± 73

14.7 ± 7.4

275 ± 141**

0.7 ± 2.2

49.1 (165/336)

36.5(230/631)

2

11.8 ± 2.9

85 ± 94

21.5 ± 10.2

368 ± 174

0.5 ± 0.2

49.0 (48/98)

36.7 (68/188)

3

13.2 ± 3.1

98 ± 102

26.6 ± 12

464 ± 221

0.6 ± 0.6

52.9 (18/34)

36.4 (24/66)

>3

12.5 ± 3.6

246 ± 198*

18.3 ± 14

1101 ± 787*

0.4 ± 0.2

75.8 (25/33)

42.9 (27/63)

*P < 0.05, vs 1,2,3; **P < 0.05, vs 2,3,4

Conclusion

Clomiphene is given for 5 days following the onset of a spontaneous or a progestagen-induced period, starting any time from days 2, 3, 4, and 5; the recommended starting dose is 50 mg/day. Letrozole has been shown to have good ovulation rate in CC-resistant PCOS women; 5 mg/day showed an optimal result, with a high monofolliculogenesis rate in PCOS women. The approach to the treatment of PCOS patients with gonadotropins is the “step-up” protocol with no difference in efficacy between the different gonadotropin preparations. PCOS patients undergoing IVF treatment have similar pregnancy, miscarriage, and live birth rates compared to those of non-PCOS patients. Ovarian stimulation in women with PCOS poses a particular challenge, as many of these women exhibit exaggerated response, resulting in an increased risk of OHSS and multiple gestations. The transfer of frozen-thawed embryos has important implications for the management of women undergoing ovarian hyperstimulation for IVF. FET endometrial preparation by following stimulation with Letrozole combined with hMG in PCOS is a novel protocol; the clinical pregnancy rate was 51.1 % (256/501), implantation rate was 36.8 % (349/948), miscarriage rate was 18 % (46/256), and live birth rate was 41.9 %(210/501). 67.1 % (336/501) cycles developed only one leading follicle (≥14 mm), and two follicles ≥14 mm developed in 19.6 % (98/501) cycles. The protocol of Letrozole combined with hMG has the potential to be the first-line treatment option for ovulation induction in PCOS women, while its use in ovarian stimulation for IVF deserves further study.

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