Introduction
Frozen embryo transfer (FET) is a widely used assisted reproduction technique that allows for the storage and utilization of excess embryos, has high clinical pregnancy and live birth rates, and reduces the risk of ovarian hyperstimulation syndrome (OHSS) ( Mackens et al., 2017 ; Veleva et al., 2009 ). The optimal endometrial preparation method in FET cycles is controversial. The most commonly used protocol is hormone replacement treatment (HRT), providing flexibility in the timing of medication and embryo transfer. In this protocol, the endometrium is artificially prepared through the administration of exogenous estrogen (E2) and progesterone.

Gonadotropin-releasing hormone agonist (GnRH-a) is a gonadotropin-releasing hormone analog, it can cause an initial flare-up effect on pituitary follicle stimulating hormone (FSH) and luteinizing hormone (LH) secretion. GnRH receptors can be downregulated by GnRH-a leading to the inhibition of gonadotropin secretion and avoiding follicular development to reduce the adverse effects on embryo implantation ( El-Toukhy et al., 2004 ); Also, GnRH-a improves pregnancy outcomes by increasing endometrial receptivity ( Orvieto et al., 2008 ). However, it remains unclear whether GnRH-a pretreatment is necessary before HRT. A randomized clinical trial indicated that HRT with GnRH-a pretreatment reduced cycle cancellation and improved clinical pregnancy and live birth rates ( El-Toukhy et al., 2004 ). However, one study suggested that removing pretreatment of GnRH-a didn’t affect pregnancy outcomes and reduced cost and adverse effects related to GnRH agonists ( Samsami, Chitsazi & Namazi, 2018 ). It is worth noting that the inclusion criteria, drug doses, and routes of administration varied among studies.

Some studies indicated that HRT + GnRH-a had benefits for specific populations. HRT-FET with GnRH-a pretreatment showed significant improvements in pregnancy outcomes in women with endometriosis or adenomyosis ( Niu et al., 2013 ; Park et al., 2016 ). This can be achieved by suppressing the proliferation of pathological lesions and reducing the effects of cytotoxic cytokines ( Khan et al., 2010 ; Tamura et al., 2014 ). Additionally, Wang et al. (2022) found that pretreatment with GnRH-a can improve live birth rates and reduce the risk of preterm birth for women with polycystic ovary syndrome (PCOS). One study found pretreatment with GnRH-a might improve the ongoing pregnancy rate in hyperandrogenic PCOS women ( Tsai et al., 2017 ). Also, HRT + GnRH-a improved the live birth rate and other clinical outcomes, especially for women with PCOS ( Xie et al., 2018 ). It is stated that the long-term deleterious effect of estrogen alone, hyperandrogenic environment, hyperinsulinemia and high LH levels can lead to abnormal endometrial receptivity ( Jiang & Li, 2022 ; Lee et al., 2019 ; Schulte, Tsai & Moley, 2015 ; Wiwatpanit et al., 2020 ). The dysregulation of the balance between proliferation and apoptosis of endometrial cells in PCOS, attributed to elevated levels of androgens, estrogens, and hyperinsulinemia, results in heightened susceptibility to alterations during cellular proliferation. Thus GnRH-a contributes to the success of FET by decreasing estrogen and androgen levels and preventing high LH levels to improve endometrial receptivity. The majority of studies have supported the utilization GnRH-a pretreatment for endometrial preparation in woman with PCOS, although some studies have argued to the contrary ( Liu et al., 2021 ; Luo et al., 2021 ).However, these dissenting studies lack a strong and rational explanation. There is still a lack of definitive conclusion about the effect of down-regulation protocol in FET cycles of women with repeated implantation failure (RIF) ( Davar, Dashti & Omidi, 2020 ; Yang et al., 2016 ). However, tubal factors account for the majority of female infertility, and depending on the research population, female infertility caused by tubal factors might be as high as 67%. Additionally, a considerable portion of infertile couples are caused by male factors including oligospermia and asthenospermia ( Carson & Kallen, 2021 ). It remains unclear whether HRT + GnRH-a improves pregnancy outcomes in these populations. The success rate of IVF usually decreases with maternal age, especially in the woman after the age of 35 and 40 ( McCoy, Nakajima & Bohler Jr, 2009 ). Few studies have explored the efficacy of HRT-FET with GnRH-a down-regulation to prepare endometrium in women of different ages, especially those of advanced age. In this research, reproductive outcomes in HRT and GnRHa-HRT will be compared in the general female population of different ages.