Association between endometrial thickness and birth weight in fresh IVF/ICSI embryo transfers: a retrospective cohort study of 9273 singleton births.

Abstract Research question What is the association between endometrial thickness (EMT) on HCG trigger day and outcomes related to birth weight in fresh IVF and intracytoplasmic sperm injection (ICSI) embryo transfer cycles? Design A retrospective cohort study of 9273 singleton live births born to women undergoing fresh IVF/ICSI cycles in a single centre between January 2014 and December 2018. Multivariable logistic regression was used to investigate the associations between EMT, low birth weight (LBW) and small for gestational age (SGA). Multivariable-adjusted linear regression models incorporating restricted cubic splines were used to investigate the dose-response relationship between EMT, birth weight and birth weight z-score, respectively. An EMT of 8 mm was set as a reference value. Results Compared with women with an EMT measuring between 8 mm or less and less than 14 mm, the risk of delivering a SGA infant was higher when EMT measured less than 8.0 mm (adjusted OR 1.78, 95% CI 1.09 to 2.90) and lower when EMT measured 14.0 mm or above (adjusted OR 0.57, 95% CI 0.35 to 0.93, respectively). Compared with women with an EMT of 8.0 mm, women with an EMT of 5.0, 6.0 and 7.0 mm were associated with a decrease of 120 g (95% CI –175 g to –66 g), 80 g (95% CI –116 g to –44 g), and 40 g (95% CI –58 g to –22 g) in birth weight; and a decrease of 0.19 (95% CI –0.27 to –0.10), 0.12 (95% CI –0.18 to –0.07) and 0.06 (95% CI –0.09 to –0.03) in birth weight z-score, respectively. Conclusions A thinner endometrium was associated with lower birth weight and birth weight z-score, and higher risk of SGA. Women with a thin endometrium warrant special attention during pregnancy.