Introduction: Based on recent studies the suggestion is that natural cycle frozen embryo transfers (NC FET) should preferably be used, with evidence suggesting that artificial cycle FET (AC FET) is subject to increased risks of adverse obstetric and perinatal outcomes and possibly lower live birth rates. There, however, is limited evidence on the most efficient and effective timing of NC FET following oocyte retrieval. Objective: In this non-inferiority randomised controlled trial, the effect on reproductive outcomes of NC FET performed immediately following the oocyte retrieval cycle (i.e., after one menstruation) will be investigated. Materials and Methods: At a single IVF centre, patients will be recruited from infertile patients presenting for freeze-all-IVF treatments. Patients aged 18 to 30 years will be enrolled, if they had ≤2 previous embryo transfers and had ≥1 blastocyst cryopreserved in their freeze-all cycles. Enrolled patients (N = 800) will be randomised (1:1) to either the immediate group (i.e., FET performed in the menstrual cycle immediately following the oocyte retrieval cycle) or the delayed group (i.e., FET performed in the menstrual cycle following two menstruations). All FET will be performed in NC. The primary outcome measure will be clinical pregnancy, defined as the visual confirmation by transvaginal ultrasound scan of a gestational sac with normal heartbeat at \>5 weeks of gestation. The analyses will be performed according to per-procedure principles. Results: The ovarian, endometrial and time to transfer outcomes of the immediate group will be compared with those of the delayed group. The clinical pregnancy rate of the immediate group will be compared with that of the delayed group.
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Clinical pregnancy from frozen embryo transfers
Timeframe: Transvaginal ultrasound examination will be performed after 5 weeks of gestation (>5.5 weeks)