Modified natural-cycle cryopreserved embryo transfer: is a washout period needed after a failed fresh cycle?

Research question: Are the characteristics of the natural cycle or modified natural cycle (mNC), or live birth rates (LBR), affected by delaying frozen embryo transfer (FET) after a failed fresh IVF cycle? Design: In a retrospective study, conducted at a university-affiliated tertiary centre, 198 women aged 18-45 years undergoing their first FET cycle after a failed fresh embryo transfer attempt using an mNC were evaluated. Cycles were divided according to the time interval between oocyte retrieval and the start of the FET cycle into the immediate FET group (<22 days) and the delayed FET group (>= 22 days). The main outcome measures were ovulation day and LBR. Results: The mean interval between oocyte retrieval and the start of the FET cycle was 15.6 +/- 3.2 days in the immediate FET group and 84.8 +/- 73.7 days in the delayed FET group (P < 0.001). Ovulation day was significantly delayed in the immediate FET group (day 171 +/- 4.4 versus day 15.4 +/- 3.7; P = 0.004). There was no difference between the immediate and delayed FET groups in terms of clinical pregnancy rate (CPR) (25.4% and 25.0%, respectively) or LBR (21.2% and 20.0%, respectively). Conclusions: Natural-cycle characteristics are similar in immediate and delayed cycles, except for a slight delay in ovulation day. Deferring mNC-FET after a failed fresh IVF cycle does not improve the reproductive outcome. These results should encourage patients and clinicians who want to proceed with FET immediately after failure of fresh IVF.