Endometrial Preparation in Assisted Reproductive Technologies

Endometrial preparation is a critical determinant of success in assisted reproductive technologies, particularly frozen embryo transfer (FET). The aim is to synchronise embryo development with a receptive endometrium, optimising implantation and pregnancy outcomes. Protocols fall broadly into natural cycles, in which endogenous hormones govern endometrial maturation, and artificial or hormone replacement cycles, where oestrogen and progesterone are administered exogenously. Modified natural cycles may include ovulation triggers such as human chorionic gonadotropin, while stimulated cycles employ mild ovarian stimulation to support corpus luteum formation. Choice of protocol influences endometrial thickness, vascularisation and the profile of secreted factors within the uterine cavity. Emerging evidence links the absence of a corpus luteum in artificial cycles with increased risks of hypertensive disorders and pre-eclampsia, suggesting that endogenous luteal support confers vascular and immunological advantages. Personalised regimens are increasingly advocated, taking account of ovarian reserve, cycle regularity and prior implantation history. Advances in hormone monitoring, endometrial receptivity assays and timing adjustments seek to refine the window of implantation, minimise early pregnancy loss and reduce obstetric complications. The global burden of infertility and the growing utilisation of FET underscore the need for evidence-based, safe and patient-centred endometrial preparation strategies.

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Endometrial Preparation in Assisted Reproductive Technologies publication trend

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Technical terms

Endometrial receptivity: The state in which the uterine lining is optimally prepared for embryo implantation.

Frozen embryo transfer (FET): The procedure of thawing and transferring cryopreserved embryos into the uterine cavity.

Natural cycle: A treatment cycle in which the woman’s own ovulatory hormones drive endometrial development.

Hormone replacement therapy (HRT) cycle: A regimen using exogenous oestrogen and progesterone to simulate the natural secretory phase.

Corpus luteum: The ovarian structure formed after ovulation that secretes progesterone and other modulators supporting early pregnancy.

Stimulated cycle: A protocol involving mild ovarian stimulation to induce follicular growth and endogenous luteal function.

References

1. Preparation of the endometrium for frozen embryo transfer: an update on clinical practices. Reproductive Biology and Endocrinology (2023).
2. Higher risk of pre-eclampsia and other vascular disorders with artificial cycle for frozen-thawed embryo transfer compared to ovulatory cycle or to fresh embryo transfer following in vitro fertilization. Frontiers in Endocrinology (2023).
3. Impact of endometrial preparation on early pregnancy loss and live birth rate after frozen embryo transfer: a large multicenter cohort study (14 421 frozen cycles). Human Reproduction Open (2022).