Endometrial Preparation in Frozen Embryo Transfer Cycles

Overview

In order to get optimal pregnancy rates after frozen embryo transfer (FET), the embryo stage and endometrium should be synchronized. Endometrial preparation is done by either natural, artificial (Hormonal replacement therapy HRT) , modified natural methods or mild ovarian stimulation. HRT cycle has a better schedualization however, there are some reports about higher rates of miscarriage, pregnancy induced hypertension (PIH) and preeclampsia (PET) in HRT cycles. A recent study has found that incorporation of the aromatase inhibitor (letrozole) to HRT cycles was associated with better FET outcomes in comparison to hormonal replacement therapy cycles alone. Meanwhile, mild ovarian stimulation protocol can be done either by oral drugs like letrozole or by letrozole plus gonadotropins . So this study aims to compare the reproductive outcomes in two endometrial preparation protocols for frozen embryo transfer cycles; letrozole mild ovarian stimulation versus HRT plus letrozole incorporation.

In order to get optimal pregnancy rates after frozen embryo transfer (FET), the embryo stage and endometrium should be synchronized. This can be done by hormonal replacement therapy (HRT), natural cycles (NC), modified natural cycles or mild ovarian stimulation. In hormonal replacement therapy cycle, estrogen and progesterone are sequentially given to resemble the hormonal course of the natural cycle . Hormonal replacement therapy cycle has a better schedualization and is used for patients with irregular cycles as polycystic ovary syndrome ( PCOS). However, there are some reports about higher rates of miscarriage, pregnancy induced hypertension (PIH) and preeclampsia (PET) in HRT cycles. A recent study has found that incorporation of the aromatase inhibitor (letrozole) to HRT cycles was associated with better FET outcomes in comparison to HRT cycles alone. Ongoing pregnancy rate (OPR) was higher in HRT plus letrozole group than HRT only group. Letrozole is a third-generation aromatase inhibitor that leads to mono-ovulatory cycles with short half-life . Miller and his colleagues found that letrozole increased Integrin expression and improved pregnancy and implantation rates among women with endometrial receptivity defects . Another study found that ovarian stimulation with letrozole was associated with increase in the expression of uterine receptivity markers including integrin, leukemia inhibitory factor, and L-selectin.