It is paramount to consider the appropriate preparation of the endometrium to receive the transferred embryos as the amount of FET cycles is increasing worldwide. Our study demonstrated that NC is superior to AC regarding pregnancy outcomes in general IVF population. To the best of our knowledge, our study is the first to evaluate the optimal endometrial preparation protocols in different age groups and concludes that there is a mild favor of AC + GnRH-a in younger women, but a remarkable priority of NC in older women.
There is no doubt that the two most important factors for a successful pregnancy are the availability of a good quality, euploid embryo, a receptive endometrium and synchrony of these factors. In FET, endometrium receptivity is achieved by dedicated endometrial preparation protocols, which can largely be divided into natural and artificial cycles. In NC, usually solely menstrual cycle monitoring is performed without any pharmacological intervention prior to ovulation, which may be spontaneous (true NC) or triggered by HCG (modified NC). However, the timing of ovulation in NC may pose scheduling difficulties, and premature ovulation may occur and increase cancellation rates. In AC, exogenous hormone is administered to prepare the endometrium for embryo implantation, with exogenous to prime the endometrium, while progesterone to complete endometrial maturation. AC with GnRH-a pretreatment offers the most control over the timing and minimize the risk of premature ovulation, but the cycle is much more prolonged and expensive.
Several studies have compared NC and AC cycles in endometrial preparation, unfortunately the results have been conflicting. Two large scale meta-analyses comparing different cycle regimens of FET failed to show a superiority of one approach over the others in terms of reproductive outcomes, however, the majority of studies included were often of a low or very low quality of evidence as most were retrospective nature with limited sample size, and some fail to report important clinical outcomes, or had a poor reporting of study methods [6, 16]. A retrospective cohort study of 1265 cycles revealed that the implantation rate was significantly higher in NC, while there were no significant differences between the groups in the clinical pregnancy, ongoing pregnancy, live birth, and miscarriages rate . El-Toukhy T and coworkers conducted a prospective randomized trial of 234 patients and found that using GnRH-a prior to exogenous steroid supplementation for endometrial preparation achieved significantly higher clinical pregnancy (24% vs 11.3%, OR 2.5, 95%CI 1.2–5.5) and live birth rates (20% vs 8.5%, OR 2.9, 95%CI 1.2-8). Our study suggests that in overall population, the incidence of live birth was higher in NC compared with AC without GnRH-a, but comparable to that of AC with GnRH-a. While the early miscarriage rate in NC was significantly lower than AC, either with or without GnRH-a pretreatment.
The decreased live birth and increased early miscarriage rate in AC might due to absence of the corpus luteum. Indeed, it has been reported that pregnancies achieved in the absence of a corpus luteum (CL) are at higher risk of hypertensive disorder, preeclampsia, and cesarean section delivery [9, 11, 18]. Relaxin is ~ 6 kDa peptide hormone secreted by CL and plays a key role in the transformation of the maternal circulation during early pregnancy especially before the “corpus luteal-placental shift” . Absence of CL results in undetectable level of relaxin, as well as decreased levels of certain angiogenic and immunoregulatory factors, leading to insufficient cardiovascular adaptation and adverse pregnancy outcomes [20, 21]. The mechanism of GnRH-a pretreatment in improving pregnancy outcomes in AC is unclear. It is proposed that long GnRH-a treatment suppress untimely rises of progesterone levels during hormonal supplementation, which may advance the endometrium and hamper pregnancy outcomes. Besides, animal study suggested that GnRH agonist up-regulated the uterine expression levels of key receptivity markers including Hoxa10, Hoxa11, Lif and integrin b3 mRNA and protein, as well as increased the abundance of pinopodes in adenomyosis, therefore restoring endometrial receptivity .
Another intriguing finding of our study is that same endometrial protocol using in different age groups results in different reproductive outcomes. In younger women, there seems to be a mild favor of AC + GnRH-a protocol as the early miscarriage rate of NC was only significantly lower than that of AC, but comparable to that of AC + GnRH-a. While the incidence of clinical pregnancy, ongoing pregnancy and live birth were both slightly higher in AC + GnRH-a than those in NC, although no reached statistical significance. Comparative proteomic analysis indicated that GnRH-a was associated with upregulation of cytoskeleton regulation and downregulation of energy metabolism on human endometrium. Younger patients present more vigorous metabolism than the older. It is possible that the downregulation of energy-metabolism proteins under GnRH-a treatment exerts a positive effect on the endometrium receptivity of younger women, but a negative effect on that of the older. In older women, the priority of NC to both AC groups was remarkable, as the ongoing pregnancy and live birth rate of NC is much higher, and early miscarriage rate was much lower in NC compared to AC, either with or without GnRH-a pretreatment. A recent study by Liu J and coworkers suggested that in women aged 38 years or over, the endometrial preparation protocols did not affect FET outcomes. However, the study was limited by the small sample size as only 457 cycles with advanced age included . Advanced maternal age is an independent risk factor of thrombotic events . Moreover, exogenous hormone increases the risk of vascular thrombosis . Premature estradiol elevation lead to apoptosis of trophoblast and is associated with uteroplacental insufficiency, hence further worsen the pregnancy outcomes in older women using AC protocols .
Strengths and limitations
This study has several strengths. First, the large sample size of 16870 cycles enhances the statistical power. Second, the clinical and laboratory practices did not substantially change over the course of study, which should minimize the possible confounders associated with pregnancy outcome. Third, we adjusted for a number of potential confounders that might otherwise have biased the findings. Last but not least, this study has good representativeness as we avoid strict inclusion and exclusion criteria. This study is mainly limited by its retrospective nature. Besides, we could not control all the confounders. Finally, as all these data come from a single fertility center, multicenter study is warranted to verify the findings.