Received August 18, 2023
Accepted February 21, 2024

Objective: The safety of assisted reproductive technology can be assessed by examining birth weight as an outcome measure. The objective of this study was to evaluate the effect of endometrial thickness during embryo transfer on newborn birth weight and preterm labor. Methods: We conducted a retrospective cohort study at the infertility department of a teaching hospital affiliated with a university of medical sciences. Eligible women were ≥18 years old and conceived a singleton pregnancy with embryo transfer and an endometrial thickness of ≥7 mm. None of the patients had diabetes, blood hypertension, and polycystic ovarian syndrome. We assessed maternal and newborn characteristics and perinatal pregnancy outcomes. Results: In total, 100 eligible patients with a mean (SD) age of 32.8 (6.2) years were included. The mean endometrial thickness during embryo transfer was 9.1 (1.2) mm, and the mean birth weight was 3040.7 (565.3) g. There were no statistically significant associations between endometrial thickness and preterm labor (P = 0.215) and between endometrial thickness and stillbirth or intra-uterine fetal death (P = 0.880). However, after adjusting for confounding factors, the association of endometrial thickness with birth weight was statistically significant [b = 124.6 (51.6), P = 0.018]. Conclusions: Within the range of ≥7 mm, endometrial thickness during embryo transfer is a predictor of newborn weight; however, it is not related to the risk of preterm labor, stillbirth, or intra-uterine fetal death.