Follicular phase length has no influence on frozen-thawed embryo transfers in natural cycles.

BACKGROUND Whether menstrual variability in women with regular and ovulatory cycle could affect embryo implantation remains controversial, with conflicting evidences presented in the literature. Thus, in this study, we evaluated the impact of prolonged follicular phase length (FPL) on the clinical pregnancy rate (CPR) after frozen-thawed embryo transfer (FET) in true natural cycles (NC). METHODS This retrospective cohort study utilized data from a large university-affiliated reproductive medicine center. Women undergoing true NC-FET were grouped as per their FPL type: Prolonged FPL (n = 127) and Normal FPL (n = 737). The primary study outcome was CPR in these 2 groups. RESULTS The FPL in the current cycle was significantly longer in the Prolonged FPL group (23.0 ± 2.4) than in the Normal FPL group (16.0 ± 2.2; p < 0.001). The crude CPR was significantly higher in the Prolonged FPL group (61.4%) than in the Normal FPL group (51.7%; p = 0.043). After adjusting for the results of potential confounders including the age, BMI, percent of optimal embryos transferred, and endometrial thickness, the difference in the CRP between the 2 groups disappeared (OR 1.28, 95% CI: 0.86-1.91, p = 0.232). No statistically significant difference was noted in the rates of implantation and miscarriage. CONCLUSIONS The current FET should not be cancelled if the ovulation time exceeds the predicted period based on the length of the previous menstrual cycle in the light of no negative effect on the pregnancy outcome.