Endometrial thickness as a predictor of ectopic pregnancy in 1125 in vitro fertilization-embryo transfer cycles: a matched case–control study

Purpose: The ectopic pregnancy (EP) rate after in vitro fertilization-embryo transfer (IVF-ET) is higher than after spontaneous conception. The reason for the increased risk of EP is not clear. We aimed to determine the risk factors associated with EP in patients undergoing IVF-ET. Methods: This was a 1:4 matched case–control study that enrolled 225 EP patients and 900 matched intrauterine pregnancy patients from the ART center of Northwest Women’s and Children’s Hospital from January 2014 to April 2018. Conditional logistic regression was used to analyze the association between risk factors and EP, and a receiver-operating characteristic (ROC) curve was generated for the predictors of EP. Results: Our findings showed that tubal factor (OR 1.61; 95% CI 1.12–2.31) and pelvic surgery other than cesarean section (OR 2.04; 95% CI 1.26–3.29) were associated with a higher risk of EP (p = 0.001). An endometrial thickness > 12 mm prior to embryo transfer (OR 0.27; 95% CI 0.13–0.56) and the number of transferable embryos (OR 0.71; 95% CI 0.65–0.78) were protective factors against EP (p < 0.001). The other factors did not have a significant effect on the probability of developing ectopic pregnancy. The area under the curve of the endometrial thickness and the number of transferable embryos for EP prediction were higher than those for tubal factor and pelvic surgery other than cesarean section. Conclusions: An endometrial thickness > 12 mm is a strong protective factor against ectopic pregnancy. Attention should be paid to women with specific characteristics who have undergone IVF-ET.