Natural versus artificial cycle for endometrial preparation in ovulatory women undergoing frozen-thawed embryo transfer: a randomised controlled trial

STUDY QUESTION: Does a natural cycle result in higher clinical pregnancy rates (CPR) with foetal heartbeat compared to an artificial cycle for frozen-thawed embryo transfer (FET) preparation in ovulatory women? SUMMARY ANSWER: The CPR with foetal heartbeat did not differ between natural and artificial cycles in ovulatory women undergoing FET. WHAT IS KNOWN ALREADY: Several protocols for endometrial preparation have been developed for the increasing number of FET cycles in reproductive medicine. In natural cycle FET (NC-FET), spontaneous ovulation is used to time the embryo thawing and transfer, while in artificial cycle FET (AC-FET), endometrial preparation involves sequential oestrogen and progesterone administration. Current evidence on pregnancy rates does not favour one regimen over the other in women with regular ovulatory cycles. STUDY DESIGN, SIZE, DURATION: We conducted a multicentre, open-label, randomised trial comparing NC-FET with AC-FET across five Belgian fertility centres. Between October 2018 and October 2024, 561 women were randomised (1:1) using a computer-generated allocation after written informed consent. The primary outcome was CPR with foetal heartbeat per cycle analysed on an intention-to-treat basis. Secondary outcomes included endometrial thickness, number of clinic visits for FET cycle monitoring and the rates of (biochemical) pregnancy, ongoing pregnancy, live birth, miscarriage, ectopic pregnancy, multiple pregnancy and cycle cancellation. Obstetric outcomes were recorded for all ongoing pregnancies. PARTICIPANTS/MATERIALS, SETTING, METHODS: Women aged 18-45 with regular ovulatory cycles and normal uterine cavities undergoing FET after vitrification and warming of one or two day-3 embryos or blastocysts were included. In NC-FET, spontaneous ovulation was detected by serial ultrasounds and serum LH and oestradiol measurements. In AC-FET, oestradiol valerate (6 mg/day) was administered from cycle day 2 (increased to 8 mg/day after 1 week if endometrial thickness was