Hemoglobin Trajectory During Pregnancy and Postpartum Hemorrhage: A Retrospective Cohort Study

Background: Studies on the association between anemia and postpartum hemorrhage (PPH) mostly use the hemoglobin (Hb) levels in a single trimester, thereby failing to capture the dynamic changes of Hb levels during pregnancy. This study aims to assess the trajectories of Hb during pregnancy and evaluate their association with PPH. Methods: The study population consisted of 4296 women who gave birth in a tertiary hospital after 28 weeks of gestation between January 2024 and February 2025. Group-based trajectory modelling was used to identify hemoglobin trajectories. Logistic regression models were applied for evaluating the associations between hemoglobin trajectories and PPH. Results: Three trajectories of maternal Hb were identified. In trajectory 1 (decline-stable, 20.41%), maternal Hb levels were higher than those in trajectory 2 in the first trimester, and rapidly declined across pregnancy with the lowest point at 35 weeks. In trajectory 2 (decline-rise, 73.16%), maternal Hb decreased during the first and second trimesters, and slightly increased during the third trimester. In trajectory 3 (stable-rise, 6.42%), maternal Hb remained stable in early and mid-pregnancy and increased in late pregnancy. Compared with trajectory 2, trajectory 1 was associated with a 2.36-fold higher risk of PPH (95% CI: 1.59– 3.47), while trajectory 3 showed no significant association (OR: 0.79, 95% CI: 0.27– 1.81), after adjustment. The area under the receiver operating characteristic curve was 0.74 (95% CI: 0.70– 0.79). Compared with women with Hb ≥ 116 g/L at 28– 32 weeks, women with Hb < 116 g/L had a 1.64-fold higher risk of PPH (95% CI: 1.08– 2.52). Conclusion: Women who are not anemic in early pregnancy but experience a rapid decline in Hb during pregnancy, especially those with Hb < 116 g/L in the early third trimester, are at high risk for PPH. Plain Language Summary: Based on the trajectory of hemoglobin levels during pregnancy, we divided the population into three groups. A total of 73.16% of women had a pattern where their hemoglobin (Hb) levels decreased during the first and second trimesters and then increased during the third trimester (trajectory 2). And 20.41% of women followed a pattern where Hb decreased more rapidly in the first and second trimester and remained lower in the third trimester (trajectory 1). Another 6.42% of women had a lower Hb in early and mid-pregnancy and an increased Hb in late pregnancy (trajectory 3). Women in trajectory 1 were 2.36 times more likely to experience postpartum hemorrhage than those in trajectory 2. Even if a woman is not anemic early in pregnancy, a rapid drop in hemoglobin later, especially Hb below 116 g/L in the early third trimester, may be a warning sign that deserves closer attention. Keywords: hemoglobin, postpartum hemorrhage, pregnancy, trajectory, group-based trajectory modelling