This study explores the effects of endometrial thickness (EMT) before embryo transfer on newborn birth weight after in vitro fertilization-frozen embryo transfer (IVF-FET). We collected the medical records related to singleton live births after IVF-FET from June 2015 to February 2019. Pregnant women were aged ≤ 42 years at delivery. Afterward, analyses were performed on outcomes related to newborns (birth weight, gestational age, delivery mode, percentage of newborns with low birth weight, and incidence of macrosomia) and pregnant women (pregnancy-induced hypertension, gestational diabetes mellitus, premature rupture of membranes, and placenta previa). The birth weight was higher in singleton newborns delivered by patients with EMT > 12 mm before embryo transfer than those delivered by patients with a thinner endometrium. The mean birth weight was 85.107 g higher in the EMT ≥ 12 mm group and 25.942 g higher in the 8-12 mm EMT group than in the EMT < 8 mm group. Independent predictors of newborn birth weight included pregnancy-induced hypertension, premature rupture of membranes, placenta previa, newborn sex, gestational age, delivery mode, number of implanted embryos, follicle-stimulating hormone levels, estradiol levels, and pre-pregnancy body mass index. The weight of newborn singletons is associated with EMT before embryo transfer in patients undergoing the first FET cycle. Specifically, the birth weight is lower for newborns delivered by patients with a thinner endometrium. Accordingly, it is warranted to increase EMT before embryo transfer for improving neonatal outcomes after fertility treatment.