High Hb concentrations across pregnancy and risks of preterm birth and low birth weight: a retrospective cohort study

To identify associations between 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 included 18 980 participants from Haidian District Maternal and Child Health Care Hospital from 2018-2021. Hb concentrations were measured at first (0-12 weeks), second (13-27 weeks) and third trimesters (≥ 28 weeks). Hb was categorised into four groups (< 110, 110-119, 120-129, ≥ 130 g/l), and Hb changes between trimesters were calculated. The latent class growth mixed model was employed to estimate Hb trajectories. Association assessment and dose-response relationship used 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). Hb changes exhibited a linear increase in odds of PTB and LBW. Women with a decline-sharp-rise trajectory had a 1·48-fold odds of PTB (OR: 1·48, 95 % CI 1·26, 1·75), compared with a decline-gradual-rise trajectory. These findings underscore the importance of monitoring Hb during pregnancy.