Effects of the Interval between Ovulation Induction using Clomiphene Citrate and Frozen Embryo Transfer on Pregnancy Outcome

Objective: To explore the best timing for frozen embryo transfer (FET) after ovarian stimulation and egg retrieval using the clomiphene citrate (CC) + human menopausal gonadotropin (hMG) ovulation induction regimen through a retrospective analysis.Methods: Data of patients who underwent CC + hMG ovulation induction and FET from January 2014 to December 2019 were analyzed retrospectively. The patients were divided into three groups according to the interval from egg retrieval to FET: CC1 (within 1 menstrual cycle), CC2 (2 menstrual cycles), and CC3 (≥ 3 menstrual cycles). Indicators such as hormone levels and pregnancy outcomes were recorded to explore the effect of different intervals on pregnancy outcome.Results: A total of 1,082 transfer cycles were included in this retrospective analysis. The implantation, clinical pregnancy, and live birth rates in the CC1 group were significantly lower than those in the CC2 and CC3 groups (P < 0.05). The E2/P4 ratio on progesterone injection day (3 days before thawed embryo transfer) was lower in the CC1 group than in the other groups (P < 0.05). After adjusting for all factors using multifactor regression analysis, the interval between egg retrieval and FET was found to be an independent predictor of the implantation, pregnancy, and live birth rates.Conclusion: An interval of more than one menstrual cycle between the day of egg retrieval after ovarian stimulation with the CC + hMG ovulation induction regimen and the day of FET can result in high implantation, clinical pregnancy, and live birth rates, which can lead to an improved pregnancy outcome.