BackgroundThe ‘Freeze all’ strategy, which consists of cryopreservation of all embryos after the ovarian stimulation has undergone extensive development in the past decade. The time required for the endometrium to revert to a prestimulation state after ovarian stimulation and thus the optimal time to perform a deferred embryo transfer after the stimulation has not been determined yet.ObjectiveTo investigate the impact of the time from oocyte retrieval to frozen-thawed blastocyst transfer (FBT) on live birth rate (LBR), obstetrical and neonatal outcomes, in ‘Freeze-all’ cycle.Materials and methodsWe conducted a large observational cohort study in a tertiary care university hospital including four hundred and seventy-four first autologous FBT performed after ovarian stimulation in ‘freeze all’ cycles. Reproductive outcomes were compared between FBT performed within the first menstrual cycle after the oocyte retrieval (‘cycle 1’ group) or delayed FBT (‘cycle ≥ 2’ group). The main Outcome Measure was the Live birth rate.Result(s)A total of 188 FBT were included in the analysis in the ‘cycle 1’ group and 286 in the ‘cycle ≥ 2’ group. No significant differences were found between FBT performed within the first menstrual cycle after oocyte retrieval (the ‘cycle 1’ group) and delayed FBT (the ‘cycle ≥ 2’ group) in terms of the live birth rate [59/188 (31.38%) vs. 85/286 (29.72%); p = 0.696] and the miscarriage rate [20/82 (24.39%) vs. 37/125 (29.60%), respectively; p = 0.413]. The obstetrical and neonatal outcomes were also not significantly different between the two groups.ConclusionOur study did not detect statistically significant differences in the LBR for FBT performed within the first menstrual cycle after oocyte retrieval versus FBT following subsequent cycles. Embryo-endometrium interaction after a FBT does not appear to be impaired by potential adverse effects of COS whatever the number of cycle between oocyte retrieval and embryo transfer.

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