High hemoglobin concentrations across pregnancy and risks of preterm birth and low birth weight: A retrospective cohort study

To identify associations between hemoglobin (Hb) concentration in different trimesters, Hb changes and Hb trajectories during pregnancy with risk of preterm birth (PTB) and low birth weight (LBW). A retrospective cohort study was conducted based on 18,980 participants from Haidian District Maternal and Child Health Care Hospital between January 2018 and December 2021. Maternal Hb concentrations were measured at first (0-12weeks), second (13-27weeks), and third trimesters (≥28weeks). Hb was categorized into four groups (<110, 110-119, 120-129, and ≥130 g/L), and Hb changes across different trimesters were calculated. The Latent class growth mixed model was employed to estimate Hb trajectories. Association assessment and dose-response relationship were estimated using multivariate logistic regression and restricted cubic spline. Compared with Hb 110-119 g/L, women with Hb ≥130 g/L significantly increased odds of PTB(OR:1.74,95%CI:1.29,2.31) and LBW(OR:2.31,95%CI:1.65,3.20) during the second trimester. In the third trimester, women with Hb 120-129 g/L showed 26% (OR:1.26,95%CI:1.04,1.51) increased odds of PTB and 72%(OR:1.72,95%CI:1.36, 2.17) increased odds of LBW, while the OR of Hb ≥130 g/L for PTB and LBW were 1.45(95%CI:1.12,1.86) and 2.60(95%CI:1.96,3.43), respectively. Furthermore, Hb changes across different trimesters exhibited a linear increase in odds of PTB and LBW. Women with a decline-sharp-rise trajectory during pregnancy had a 1.48-fold odds of PTB(OR:1.48,95%CI:1.26,1.75), compared to those with a decline-gradual-rise trajectory. In conclusion, elevated Hb (≥120 g/L) in mid-to-late gestation as well as insufficient Hb decline or abnormal Hb trajectory, were associated with increased odds of PTB and LBW. These findings underscore the importance of monitoring Hb during pregnancy.