Effect of endometrial thickness on the risk of ectopic pregnancy for frozen embryo transfer

Objective:Few studies have explored the suitability of endometrial thickness(EMT)as a predictor of ectopic pregnancy(EP)in frozen embryo transfer(FET)cycles.Our study aimed to investigate the association between EMT and EP and explore the cutoff value of EMT to predict EP.Methods:This retrospective cohort study included women aged 20-45 years undergoing FET between March 2015 and September 2021.The primary outcomes and measured variables were EP and EMT,respectively.The threshold effect of EMT on EP was calculated based on a constructed smooth curve fitting.The patients were divided into two groups based on the threshold for comparison.A subgroup analysis was conducted to examine whether the relationship between EMT and EP remained consistent across the different subgroups.Additionally,a generalized linear model was used to analyze the interactions between the treatment factors and EMT in EP.Results:In total,27,549 FET cycles were included,with 27,086 intrauterine and 463 ectopic pregnancies.The risk of EP in the EMT≤8.7 mm group was 3.36 times higher than that in the EMT>8.7 mm group(odds ratio[OR]:3.36,95%confidence interval[CI]:2.57-4.05).Subgroup analysis further indicated that a thin EMT was a risk factor for EP,independent of other risks.We also noted that diminished ovarian reserve,tubal factor infertility,transfer of two embryos,or transfer of cleavage-stage embryos may further enhance the risk of EP in patients with thin endometrium.Conclusions:Thin endometrium was associated with an increased risk of EP.Patients with EMT less than 8.7 mm need close medical observation and management after embryo transfer.In addition,diminished ovarian reserve,tubal factor infertility,transfer of two embryos,or transfer of cleavage-stage embryos may significantly enhance the effect of a thin endometrium on the risk of EP.