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

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 of 8.0 to < 14.0 mm the risk of delivering a SGA infant was increased with EMT < 8.0 mm (adjusted odds ratio [aOR] 1.78, 95% confidence interval [CI] 1.09 to 2.90) and decreased with EMT >= 14.0 mm (aOR 0.57, 95% CI 0.35 to 0.93, respectively). Birth weights of infants born to women with an EMT of 8.0 mm compared with women with EMT of 5.0, 6.0, and 7.0 mm were lower by 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) respectively; infant birth weight z-scores were also decreased by 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), 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.