Decreased endometrial thickness between HCG day and embryo transfer day improves the pregnancy rate of IVF/ICSI cycles

Abstract Background Previous studies have suggested that pregnancy outcome and endometrial thickness before embryo transfer are closely related. However, we found that endometrial thickness changed even within a few days between HCG day and embryo transfer day. There have been few studies on the relationship between endometrial thickness change (ETC) and pregnancy outcomes. The aim of this study was to assess whether the ETC from HCG trigger day to embryo transfer day affects pregnancy outcomes of in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) cycles. Methods This was a retrospective single-center cohort study. In total, 9773 IVF/ICSI embryo transfer cycles involving the long protocol at Chongqing Reproductive and Genetics Institute from January 1st, 2016, to December 30th, 2019, were included. The 9773 cycles were divided into 2 groups based on whether the endometrial thickness decreased from HCG day to embryo transfer day, and clinical pregnancy rates (CPRs) and ongoing pregnancy rates (OPRs) were compared. Logistic regression was used to determine whether an ETC was an independent risk factor for CPR or OPR. Results The CPR and OPR of the decreased endometrial thickness group were higher than those of the non-decreased endometrial thickness group (62.65% and 52.6% vs. 60.56% and 50.18%) (p = 0.041 vs. 0.027). The endometrial thickness change was an independent risk factor for CPR (OR = 0.920, P = 0.003, 95% confidence interval 0.870–0.972) and OPR (OR = 0.907, P = 0.000, 95% confidence interval 0.859–0.958) Conclusions A proper decrease in endometrial thickness from HCG trigger day to embryo transfer day in IVF/ICSI cycles increases the CPR and OPR.