Associations of trimester–specific maternal lipid levels with abnormal foetal birth weights: a cohort study

Pregnancy triggers substantial increases in maternal lipids, which are essential for foetal growth but are linked to severe adverse outcomes when dysregulated. Despite evidence linking dyslipidaemia to birth weight abnormalities, inconsistent findings and a lack of standardized management criteria persist. A cohort study involving 2,032 participants was conducted in Beijing, China. The associations between maternal lipid profiles across early, middle, and late pregnancy and foetal birth weight outcomes were assessed by multivariate logistic regression, chain mediation models, and restricted cubic spline (RCS) analysis. Multivariate analysis demonstrated that third–trimester total cholesterol (TC), high–density lipoprotein cholesterol (HDLC), and low–density lipoprotein cholesterol (LDLC) levels were significantly inversely associated with the risk of small for gestational age (SGA) outcomes (all ORs < 1, P < 0.05). Conversely, third–trimester triglyceride (TG) levels were significantly positively associated with large for gestational age (LGA) outcomes ( OR = 1.24, 95% CI : 1.05–1.47, P < 0.05). Additionally, late–pregnancy HDLC and LDLC levels were protective against LGA outcomes (adjusted OR = 0.44 and 0.64, respectively; P < 0.05). Chain mediation analysis revealed that early and mid–pregnancy lipids were not directly associated with birth weight but were significantly associated with outcomes through indirect pathways mediated by late–pregnancy lipids (TG_1→TG_3→LGA, β = 0.073; TC_1→TC_3 →SGA, β = − 0.122). Furthermore, the RCS analysis revealed a nonlinear relationship between third–trimester TG levels and birth weight with a saturation threshold of 4.88 mmol/L. Below this level, birth weight increased positively with increasing TG concentration. Late–pregnancy lipid profiles exhibit the strongest direct associations with birth weight deviation. Specifically, elevated TC, HDLC, and LDLC levels in the third trimester are protective against SGA outcomes, whereas TG in the third trimester is nonlinearly associated with birth weight. Crucially, early and mid–pregnancy lipids are indirectly associated with these outcomes, suggesting that trimester–specific monitoring and tailored interventions throughout gestation may be important for optimizing foetal growth.