In this study, we found that higher progesterone concentration and thinner EMT on the day of hCG administration were associated with the decreased clinical pregnancy rate and live birth rate in PCOS patients undergoing their first single blastocyst frozen transfer. QUICKI < 0.357 and fewer oocytes retrieved was associated with miscarriage in univariate analysis but lost statistical significance after adjusting for confounders. In addition, more AFC was a risk factor for preterm birth.
Studies showed that increased serum progesterone on the day of hCG administration had a detrimental effect on pregnancy outcomes in fresh embryo transfer cycles (18–20), and it was also indicated that in patients with PCOS, elevated progesterone on the hCG trigger day was associated with reduced clinical pregnancy rate in fresh IVF cycles (21). This may be attributed to the fact that abnormally high progesterone values during a fresh cycle can provoke a premature conversion of the endometrium to the secretory phase and an early closure of the implantation window (22, 23). Additionally, it was reported that in FET cycles, clinical pregnancy rates in patients with high progesterone in fresh cycles were not significantly different from those with normal progesterone concentration (23, 24). Therefore, physicians often propose frozen embryo transfer to address the embryo-uterine asynchrony in patients with elevated progesterone concentration in the ovarian stimulation cycles. However, it appeared that pregnancy outcomes in frozen cycles were also affected by high progesterone levels. A study from Hong Kong reported that elevated progesterone levels lasting two days or more before the LH surge were associated with reduced clinical pregnancy rates in patients undergoing FET-natural cycles, and women with elevated progesterone levels in ovarian stimulation cycles also tend to have elevated progesterone levels in subsequent natural cycles (25). Kofinas et al. reported that in patients undergoing HRT-FET cycles, progesterone levels greater than 20ng/dl on the day of embryo transfer were associated with reduced live birth rate (26). In the PCOS patients undergoing HRT-FET cycles included in our study, elevated progesterone concentration on the hCG trigger day remained associated with lower clinical pregnancy and live birth rate.
Elevated serum progesterone on the hCG trigger day may derive from the accumulation of progesterone production from multiple follicles during ovarian stimulation for IVF treatment (27, 28). Generally, the mechanisms of adverse pregnancy outcomes caused by elevated progesterone on hCG trigger day may be two aspects, one is oocyte damage and the change of embryo quality, and the other is the impairment of endometrial receptivity. There is still no consensus on the effect of elevated progesterone on embryo quality. In a donor oocyte programme, no significant difference in the pregnancy rate was found between the recipients who received oocyte donated by patients with and without elevated progesterone on the day of hCG administration (29). Turgut et al. found that elevated progesterone had no negative effect on embryological parameters of blastocysts (30). However, two large retrospective studies indicated that elevated serum progesterone on the day of oocyte maturation was associated with a lower rate of high-quality blastocyst formation (31, 32). In GnRH antagonist IVF/ICSI cycles, high serum progesterone levels on the hCG trigger day were associated with reduced embryo utilization and cumulative live birth rates (33). Currently, limited studies have been conducted on impaired oocyte quality associated with high progesterone in IVF cycles, and the mechanisms involved are unclear. Some animal studies showed that lower follicular progesterone concentrations improve bovine oocyte development in vitro (34), and that oocyte maturation and developmental capacity were regulated by progesterone-responsive genes (31, 35, 36). In this study, the oocytes collected during stimulation cycles in patients with PCOS may be compromised by high progesterone, which led to impaired embryo quality and reduced high-quality embryos, affecting the pregnancy outcome in frozen embryo cycle.
Most studies supported that elevated progesterone had negative effects on endometrial microenvironment (24, 37, 38). In antagonist IVF cycles, progesterone receptors on the endometrium were up-regulated on the day of hCG administration, and the effect of progesterone was amplified, leading to premature luteinization and increased endometrial maturation (39). Liu et al. reported that high levels of progesterone before oocyte retrieval impaired components of the NK cell-mediated cytotoxic pathway in the endometrium (19). Although the premature luteinization (progesterone elevation) of the endometrium could be resolved in HRT-FET cycles to a large extent (40), endometrial gene expression and epigenetics was abnormally altered by exposure to high progesterone levels on hCG trigger day (37, 38), resulting in impaired endometrial receptivity and may even influence pregnancy outcomes in the subsequent FET cycles. Overall, a combined effect of compromised embryo quality and impaired endometrial receptivity may be responsible for the reduced clinical pregnancy and live birth rates in FET cycles in PCOS patients with high progesterone in this study. Therefore, in view of the detrimental effects of high progesterone on pregnancy in fresh cycles, a freezing strategy is not sufficient, and it was suggested that the total dose of ovarian stimulation should be reduced in order to establish a balance between the low value of progesterone and the high number of oocytes to improve the pregnancy outcomes (33). Meanwhile, when high progesterone levels are observed in fresh IVF cycles in patients with PCOS, physicians should regularly monitor progesterone levels and inform patients of the potential risk of adverse pregnancy outcomes in FET cycles.
Endometrial thickness is the most commonly used indicator to evaluate endometrial receptivity in clinical practice (41). Previous studies have shown that thin endometrium has adverse effects on pregnancy outcomes in fresh cycles (42–45). It was also reported that endometrial thickness was positively correlated with clinical pregnancy rate or live birth rate in frozen embryo transfer cycles (46–48), and similar findings was observed in patients with PCOS (49, 50). Our study showed that thinner endometrium on hCG trigger day was associated with reduced clinical pregnancy and live birth rates in PCOS patients undergoing their first FET cycle. This is consistent with Basir's study in which they found that the group with lower endometrial thickness in stimulated cycles also had lower pregnancy rates in frozen and natural cycles, and that there was a strong correlation between endometrial thickness in stimulated and natural cycles, suggesting that thinner endometrium in the first IVF cycle may be difficult to improve in subsequent natural and frozen cycles (51). This may be attributed to endometrial insensitivity resulting in poor endometrial development and may be associated with some intrinsic uterine pathology (51, 52), as well as defects in endometrial estrogen and progesterone receptors that may exist in impaired developed endometrium (53, 54).
Studies have reported reduced endometrial receptivity in PCOS patients (55, 56), which may be caused by endocrine and metabolic disorders such as progesterone resistance, androgen or insulin elevation (57, 58). A significant negative correlation between endometrial thickness and serum total testosterone was observed in PCOS patients (59). The mechanisms for implantation failure or pregnancy loss due to thin endometrium have not been fully elucidated. The thinner functional layer may expose the embryo to higher oxygen concentrations in the basal endometrium, which is unfavorable for fetal growth (60). Other studies have suggested that abnormal transcriptional changes in thin endometrium were also involved in pregnancy failure (61, 62). Our study suggested that PCOS patients with thinner endometrium in their first IVF cycle may remain at a higher risk of pregnancy failure in FET cycles compared to patients with thicker endometrium. Active treatments are needed to increase endometrial thickness, such as administration of estrogen (63), low-dose aspirin (64), vaginal sildenafil (65), intrauterine infusion with granulocyte colony-stimulating factor (G-CSF) (66) or platelet-rich plasma (PRP) (67, 68). However, there is a lack of conclusive evidence to prove the effectiveness of these methods, and PCOS patients with thin endometrium detected in the IVF cycle should be informed that the pregnancy outcome may remain unsatisfactory even in the frozen embryo cycle.
Previous studies have found an increased risk of preterm birth in women with PCOS during natural pregnancy and IVF cycles (5, 69–73). Our study identified the higher AFC as a risk factor for preterm delivery in patients with PCOS undergoing their first FET, which to our knowledge has not been reported before. A recently published retrospective study of 4266 live birth cycles showed that AFC of more than 24 (OR = 1.378, 95% CI: 1.035–1.836) was an independent risk factor for preterm delivery in patients of IVF cycles (74). Currently, there are few data on risk factors of preterm delivery in women with PCOS. The mechanism of increased risk of preterm delivery in PCOS women is not very clear, which may be involved with hyperandrogenism, glucose and lipid metabolism disorders (70, 75). Excess androgen may lead to alterations in cervical remodeling and myometrial function (76), and even result in myometrial relaxation through non-genomic actions (77, 78). Abnormally increased androgens may also promote preterm birth by increasing the risk of gestational diabetes mellitus (77, 79). Studies reported a positive association between follicle number and androgen levels in women with PCOS (80), which may explain the association of more AFC with preterm birth. Our findings suggest that AFC may be a more sensitive factor than androgen level in predicting preterm delivery in FET cycles. AFC is a common indicator to evaluate women's ovarian reserve and the response to ovarian stimulation in the ART cycle (81–83). It was found that PCOS patients with fewer AFCs during IVF cycles are more likely to conceive, possibly due to the higher responsiveness to ovarian stimulation in patients with higher AFC, leading to premature luteinization and impaired endometrial receptivity (21). A study reported that AFC was a risk factor for clinical pregnancy loss in women with PCOS undergoing IVF cycles (84). These findings suggested that high AFC had a negative effect on pregnancy outcomes in PCOS patients undergoing IVF. Overall, the effect of AFC on the risk of preterm delivery in PCOS patients undergoing FET cycles still needs more data to explore.
QUICKI is a common indicator of insulin sensitivity, and based on previous studies (17, 85), we classified patients with QUICKI values below 0.357 as the insulin resistance group. Our univariate regression analyses showed that QUICKI less than 0.357 and a smaller number of oocytes retrieved were associated with miscarriage. However, after adjusted for relevant variables such as age and BMI, the effects of QUICKI less than 0.357 and number of oocytes on miscarriage were no longer statistically significant. In our additional analyses, we found that PCOS patients with higher BMI were more likely to have QUICKI values below 0.357 (OR = 1.385, 95% CI 1.232–1.556, P < 0.001), suggesting patients with higher BMI would be more susceptible to insulin resistance. Obesity was often accompanied by insulin resistance and they were both common metabolic symptoms in patients with PCOS (86, 87). Studies reported that higher BMI was a risk factor for pregnancy loss in patients with PCOS (84, 88). Therefore, our data are insufficient to support that insulin resistance could perform an independent role in miscarriage in PCOS patients, and more data may be required for stratification studies with different ranges of BMI.
Studies showed that the number of oocytes retrieved was associated with increased cumulative live birth rate (89–91), and patients with a higher number of oocytes retrieved tended to have more options for embryo transfer, and were theoretically more likely to have good quality embryos transferred (89). The quality of the embryo transferred may explain the result of our univariate analysis where patients with fewer oocytes retrieved experienced higher miscarriage rate. We further analyzed the correlation between the number of oocytes retrieved and the rate of high-quality embryo transferred, and a positive association was found between them (OR = 1.128, 95% CI 1.034–1.231, P = 0.007). Consequently, the effect of the number of oocytes retrieved on miscarriage disappeared after adjusted for the embryo quality. In addition, it’s noted that PCOS patients usually had ovarian hyperresponsiveness and may easily obtain more oocytes, which warranted prevention of the risk of OHSS and thrombosis associated with increased oocyte numbers (91). Therefore, the number of oocytes retrieved was suggested to be controlled to an optimal range in patients with PCOS (91, 92).
This study was concerned with the factors influencing pregnancy outcome in PCOS patients undergoing their first FET cycle. Our inclusion criteria were strict and excluded some patients with underlying diseases, thus improving homogeneity and comparability among patients. Additionally, the conclusions were more reliable after controlling confounders using multivariate logistic regression analysis. However, there are still several limitations in this study. First, the sample size was relatively small, which may account for the fact that some of important factors affecting pregnancy outcomes in women with PCOS, such as age and BMI, were not confirmed in our study. Moreover, the limited sample size also did not allow for further analysis on the specific thresholds and predictive modelling of the contributing factors in our study. Second, only PCOS patients who underwent their first FET were included, and the ovarian stimulation protocol and endometrial preparation protocol adopted by patients were controlled for better homogeneity, which also reduced the generalizability of our findings.