Background <p>Most prior studies on anemia in pregnancy, preterm birth (PTB), and non-reassuring fetal status (NRFS) use only single-trimester hemoglobin concentrations and fail to capture gestational hemoglobin changes. This study aimed to assess the associations of pregnancy hemoglobin trajectories with these two adverse perinatal outcomes.</p> Methods <p>A retrospective cohort study was conducted using pregnancy records from Longquanyi District of Chengdu Maternity and Child Health Care Hospital (from July 2024 to June 2025) , involving 2,954 pregnant women with complete serial hemoglobin measurements. The outcome measures were PTB and NRFS. Latent growth mixture modeling was applied to identify distinct hemoglobin trajectories during pregnancy. Binary logistic regression was constructed to explore the association between hemoglobin trajectories and birth outcomes.</p> Results <p>The incidence of anemia during pregnancy was 29.8% in 2954 pregnant women. Three distinct hemoglobin trajectories were identified: Traj-1 (high-rapid decline), Traj-2 (moderate-gradual decline), and Traj-3 (low-delayed recovery). Compared with the Traj-2 group, both the Traj-1 (adjusted odds ratio (aOR) = 1.780, 95% confidence interval (CI) 1.144, 2.769) and Traj-3 (aOR = 2.051, 95% CI 1.084, 3.881) groups were significantly associated with PTB. Moreover, the Traj-3 (aOR = 2.580, 95% CI 1.223, 5.440) group exhibited an increased risk of NRFS.</p> Conclusion <p>This study suggests that specific longitudinal hemoglobin trajectories, particularly high-rapid decline and low-delayed recovery patterns, exhibited an increased risk of PTB and NRFS, highlighting the clinical value of dynamic monitoring over single-point assessment in the practice of maternal and child health care.</p>