Objective To investigate the associations between body mass index (BMI) trajectories in early and mid-pregnancy and preterm birth, to examine the mediating effects of gestational diabetes mellitus (GDM) and inflammatory markers, and to provide evidence for early prevention and intervention of preterm birth.

Methods A prospective cohort study was conducted and 1221 pregnant women who delivered babies at Chongqing Maternal and Child Health Hospital between 2018 to 2021 were included. Group-based trajectory modeling was used to identify BMI trajectories in early and mid-pregnancy. Logistic regression and causal mediation analyses were performed to evaluate the mediating effects of GDM and inflammatory factors in the associations between BMI trajectories and preterm birth.

Results TThree BMI trajectories were identified, and the participants were categorized accordingly into Group 1 (48.98%), Group 2 (38.41%), and Group 3 (12.61%). Group 2 demonstrated a trajectory similar in shape to that of Group 1, with an overall level between those of Group 1 and Group 3. Participants in Group 3 were in the obese range in early pregnancy, with their BMI decreasing slightly along with the progression of pregnancy and stabilizing and then rising slightly in mid-pregnancy. After adjusting for confounders, Group 3 showed higher risks of preterm birth, which were 3.1 times those of Group 1 (odds ratio OR = 3.10, 95% CI: 1.59-6.06). Additionally, white blood cell count and monocyte percentage significantly mediated this association, with proportion eliminated of 0.5% and 5.1%, respectively. No mediating effect of GDM was observed.

Conclusion Pregnant women with higher BMI during early and mid-pregnancy exhibit a significantly increased risk of preterm birth. White blood cell count and monocyte percentage may serve as key inflammatory mediators linking this association while the mediating effect of GDM is not found.