Progesterone levels on the day of blastocyst transfer in modified natural cycle frozen embryo transfer cycles: a sub-study of an ongoing randomised controlled trial

Research question Are serum progesterone levels on the day of modified natural cycle (mNC) frozen blastocyst transfer (FET) without luteal phase support (LPS) associated with clinical pregnancy rate (CPR)? Design Data was collected between January 2019 and October 2022 as a sub-study of an ongoing randomised controlled trial (RCT) assessing pregnancy outcomes in mNC-FET. The sub-study included all women (N=209) randomized to mNC-FET without LPS at the time of data extraction. Participants were aged 18-41 years, had regular menstrual cycles, and underwent mNC-FET treatment with single blastocyst transfer. Associations between serum progesterone level on day of blastocyst transfer and CPR, pregnancy rate (PR), and pregnancy loss rate (PLR) was examined between groups with low and higher progesterone levels using the 25th and the 10th percentiles as cut-offs. Multivariate logistic regression analyses were performed to adjust for potential confounding factors. Results Progesterone levels on the day of blastocyst transfer in mNC-FET without LPS ranged from 4.9-91.8 nmol/L, with the 25th and 10th percentile at 29.0 nmol/L and 22.5 nmol/L, respectively. Serum levels of progesterone did not differ between women with or without a clinical pregnancy (mean (SD): 38.5 (14.0) vs 36.8 (12.4) nmol/L, p=0.350). Further, the CPR, PR and PLR were similar in women with low or high progesterone levels when using the 25th or the 10th progesterone percentile as cut-off. Multivariate regression analyses showed no association between progesterone levels and CPR. Conclusion We found no association between progesterone levels on the day of blastocyst transfer and pregnancy outcome in women undergoing mNC-FET without progesterone LPS.