Association between endometrial thickness and birthweight of singleton infants from frozen-thawed embryo transfer cycles: a retrospective cohort study

Research Question What is the association between endometrial thickness (EMT) and birthweight of singleton infants from frozen-thawed embryo transfer cycles? Design This retrospective cohort study was conducted in the center for reproductive medicine affiliated in Shandong University from January 2016 to December 2019. Participants were categorized into natural cycle (NC, n=8132) group and hormone replacement therapy (HRT, n=4975) group, respectively. The primary outcomes were the measures of birth weight, including absolute mean birth weight, birth weight Z-score, low birth weight, small for gestational age (SGA), large for gestational age (LGA), and preterm birth. Multivariable logistic regression and multivariable-adjusted linear regression models that incorporated restricted cubic splines (RCS) were used. Results In the HRT group, the risk of delivering an SGA infant was increased in women with EMT < 8.0 mm compared with women with an EMT of 8.0 to < 12.0 mm (aOR 2.01, 95%CI 1.28-3.15), and increased with EMT ≥ 12.0 mm (aOR 1.82, 95%CI 1.02-3.26). Furthermore, compared with women with an EMT of 8.0 to < 12.0 mm, the risk of delivering an LGA infant was decreased with an EMT < 8.0 mm (aOR 0.69, 95%CI 0.55-0.87). An inverted U-shaped relationship was found between EMT and birth weight in women with HRT. No significant differences were among singletons in the NC group. Conclusion An inverted U-shaped nonlinear association was observed between continuous EMT and neonatal birth weight in FET cycles. Neonates born from HRT cycles were associated with increased birth weight and a higher risk of LGA than those born from NC.