Comparison of different endometrial preparation protocols on frozen embryo transfer pregnancy outcome in patients with normal ovulation.

RESEARCH QUESTION: What is the effect of letrozole use in patients undergoing frozen embryo transfer (FET) with normal ovulation? Although the number of FETs is increasing, an optimal protocol for FET (particularly vitrified-warmed embryo transfer) is yet to be determined. The aim of this study was to evaluate letrozole use on patients with normal menstrual cycles compared with hormone replacement therapy (HRT) cycles and natural cycles. DESIGN: The study involved 2849 patients. Patients were divided into three groups: HRT cycle (n = 2115), letrozole cycle (n = 532) and natural cycle (n = 202). Inverse probability of treatment weighting aimed to equate each group according to measured baseline covariates to achieve a comparison with reduced selection bias and live birth rate as main pregnancy outcome was analysed. RESULTS: In the crude analysis, the letrozole group had a higher live birth rate compared with the HRT cycle (OR 1.18, 95% CI 1.06 to 1.33) and natural cycle (OR 1.24, 95% CI 1.11 to 1.41); after adjusting for confounding factors, live birth rate was consistently higher in the letrozole group. Moreover, the biochemical pregnancy, clinical pregnancy, ongoing pregnancy and full-term delivery rates were higher in the letrozole group. CONCLUSION: For infertile women with normal menstrual cycle undergoing FET, mildly stimulated cycles with letrozole present a relatively large advantage compared with HRT cycle and natural cycle, with higher live birth pregnancy, indicating that letrozole administration could improve pregnancy outcomes in this population.