Is there an effect of the start day of luteal phase support on pregnancy outcomes in natural vitrified-warmed blastocyst transfer?

To evaluate whether the timing of luteal phase support (LPS) initiation affects pregnancy outcomes in natural cycle vitrified-warmed blastocyst transfer (NC-FET) cycles. This retrospective cohort study included NC-FET cycles performed between January 2022 and December 2024 at a tertiary university fertility center. Only true natural cycles with blastocyst transfer were analyzed. Patients were divided into two groups according to the initiation of LPS: two days before embryo transfer (Group 1) or on the day of embryo transfer (Group 2). All patients received vaginal and subcutaneous progesterone for LPS. Baseline characteristics, embryo features, serum progesterone levels on transfer day, and reproductive outcomes were compared. Multivariate logistic regression and receiver operating characteristic (ROC) analyses were performed to identify factors associated with clinical pregnancy. A total of 246 NC-FET cycles were analyzed (Group 1: n = 172; Group 2: n = 74). Baseline characteristics, embryo quality, and transfer parameters were comparable between groups. Serum progesterone levels on the day of transfer were significantly higher in Group 1 (32.1 ± 13.0 ng/mL) compared with Group 2 (10.3 ± 3.8 ng/mL; p = 0.001). However, total pregnancy rate (66.2% vs. 47.1%, p = 0.006), clinical pregnancy rate (56.8% vs. 40.7%, p = 0.020), and live birth rate (52.7% vs. 37.2%, p = 0.024) were significantly higher in Group 2. Logistic regression analysis showed that initiation of LPS on the day of transfer was independently associated with increased clinical pregnancy (OR 2.37, 95% CI 1.17–4.81). Serum progesterone level did not predict pregnancy outcomes. In NC-FET cycles, initiating luteal phase support on the day of embryo transfer is associated with improved pregnancy and live birth rates compared with earlier initiation, despite lower serum progesterone levels.