Number of top-quality embryos transferred has no advantage in women over the age of 40 in FET cycles.

INTRODUCTION: The aim of the study was to explore the impact of the clinical and embryological factors on the pregnancy outcome of FET cycle, while determining the applicability range for the primary predictor. METHODS: A total of 4395 FET cycles (2986 couples) were included in this retrospective study. A bootstrapping stepwise variable selection algorithm was used to identify independent predictors of the clinical pregnancy rate (CPR) from 24 clinical and embryological variables. Multivariate logistic regression was carried out to assess the impact of these predictors. The primary predictor was stratified to ascertain their applicability. RESULTS: The final multivariate model incorporated the following 10 independent predictors: number of top-quality embryos transferred, age, number of in vitro fertilization/intracytoplasmic sperm injection attempts, endometrial thickness on trigger day, whether the embryo was cultured after thawing, fresh embryo transferred prior to FET, number of FET attempts, post-thaw embryo blastomere integrity, embryo developmental stage at transfer, and duration of infertility. The number of top-quality embryos transferred was found to be one of the most important predictors of pregnancy. For the younger age groups (≤ 30, 31-35 and 36-40 years), there was a significant increase in the CPR and live birth rate when more than one top-quality embryo was transferred. However, no significant differences were observed in the CPR between those with no or only one top quality embryo and those with two or more top-quality embryos transferred (14.0% vs. 18.3%, respectively; p = 0.553) in the group aged > 40 years. CONCLUSIONS: Our data reveal that the number of top-quality embryos transferred is one of the most important factors in predicting the pregnancy outcomes of FET. Further, the number of top-quality embryos transferred showed no benefit for women > 40 years in the FET cycles.