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.