A thinner endometrium is associated with lower newborn birth weight during in vitro fertilization–frozen-embryo transfer: cohort study

Background: The purpose of this study is to explore the influence of endometrial thickness (EMT) before embryo transfer on birth weight after in vitro fertilization–frozen-embryo transfer (IVF–FET).MethodsThis was a retrospective cohort study. We collected the medical records associated with singleton live births from Tianjin Central Obstetrics and Gynecology Hospital from June 2015 to February 2019 after IVF–frozen-embryo transfer (FET). Patients were ≤ 42 years at delivery. Outcomes related to newborns were birth weight, gestational age, delivery mode, low birth weight, and prevalence of macrosomia. Outcomes related to pregnant women were gestational hypertension, gestational diabetes mellitus, premature rupture of membranes and placenta previa.ResultsThe birth weight of singleton newborns was higher for newborns delivered by patients with EMT > 12 mm before embryo transfer than newborns delivered by patients with a thinner endometrium. Regression analysis showed that the EMT ≥ 12 mm group had a gain in mean birth weight of 85.107 g compared with that in the EMT < 8 mm group, whereas the group with EMT of 8–12 mm had an increase in mean birth weight of 25.942 g compared with that in the EMT < 8 mm group. Hypertension during pregnancy, premature rupture of membranes, placenta previa, newborn sex, gestational age, delivery mode, number of implanted embryos, follicle-stimulating hormone (FSH) level, estradiol (E2) level, and pre-pregnancy body mass index (BMI) were all independent predictors of newborn birth weight. The regression model for predicting the newborn birth weight was: Y (birth weight) = 25.942×(EMT of 8–12 mm) + 85.107×(EMT > 12 mm) + 123.483×(hypertension during pregnancy) + 148.859×(premature rupture of membranes) + 182.342×(placental position) − 126.242×(newborn sex) + 23.837×(number of days of pregnancy) + 130.487×(delivery mode) − 55.023×(number of implanted embryos) − 6.215×FSH level − 1.124×E2 level + 22.218×BMI − 4468.101.Conclusion(s)EMT before embryo transfer in patients undergoing their first freeze–thaw embryo transfer cycle is related to the weight of newborn singletons. The newborn birth weight for patients with a thinner endometrium is lower. EMT should be increased before embryo transfer to improve neonatal outcomes after fertility treatment.