Associations of maternal hemoglobin levels with fetal biometrics and adverse birth outcomes: A prospective study in China

Maternal hemoglobin and fetal growth change dynamically throughout pregnancy. We examined the associations of time-specific hemoglobin levels and hemoglobin trajectories with fetal biometrics and adverse birth outcomes. This prospective study included 6,844 pregnant women (mean age 26.6 ± 3.7 years) from the Tongji-Huaxi-Shuangliu Birth Cohort. Hemoglobin levels were measured at four periods: early (6-12 gestational weeks), middle (13-27), middle-late (28-32), and late pregnancy (33-37). Fetal biometrics were assessed by ultrasound from middle to late pregnancy. Birth outcomes were obtained from medical records, including small for gestational age (SGA), low birth weight (LBW), and preterm birth. Three hemoglobin trajectories were identified: consistent decline (Trajectory 1), consistently low (Trajectory 2), and increase from middle-late pregnancy (Trajectory 3). Compared with Trajectory 1, Trajectory 3 was associated with lower estimated fetal weight (β, -0.54; 95% confidence interval (CI), -0.99, -0.09) and abdominal circumference (β, -0.21; 95% CI, -0.40, -0.01) in late pregnancy, and higher umbilical artery resistance index across pregnancy (0.65; 0.31, 1.00). Trajectory 3 was also associated with higher risk of LBW (odds ratio, 1.57; 95% CI, 1.09, 2.26). In middle-late pregnancy, higher hemoglobin (≥130 g/L) was associated with higher risks of LBW (odds ratio, 2.26; 95% CI, 1.08, 4.25) and preterm birth (2.03; 1.12, 3.44) compared with the reference (110-129 g/L). Elevated maternal hemoglobin from middle-late pregnancy onward may be associated with lower fetal weight and increased risk of LBW. Dynamic monitoring of hemoglobin, with targeted assessment of iron status in women with elevated hemoglobin levels, may inform nutritional management during pregnancy.