Trimester-specific associations of maternal vitamin D metabolic markers with risks of small vulnerable newborns: a prospective birth cohort study

Vitamin D metabolites beyond total 25(OH)D may influence fetal development, yet their trimester-specific associations with small vulnerable newborns (SVN) remain underexplored. This study examined longitudinal associations of multiple vitamin D metabolic markers with SVN-related outcomes and identified key determinants during pregnancy. In this prospective cohort of 2,505 mother-infant pairs from China, serum levels of five vitamin D metabolites 25(OH)D 2 , 25(OH)D 3 , total 25(OH)D, 3-epi-25(OH)D 3 , and 24,25(OH) 2 D 3 ] were quantified using HPLC-MS/MS. Two vitamin D metabolite ratios (VMR), VMR1 [24,25(OH) 2 D 3 /25(OH)D 3 ] and VMR2 [3-epi-25(OH)D 3 /25(OH)D 3 ], were derived. Outcomes included low birth weight (LBW), preterm birth (PTB), small for gestational age (SGA), and composite SVN. Associations were evaluated using multivariable logistic regression and restricted cubic spline models. Determinants, including dietary intake, supplementation, season of sample collection, and sociodemographic factors, were evaluated via multivariable linear regression. Vitamin D deficiency decreased from 42.0% in the first trimester to 13.8% in the third trimester. Season and vitamin D intake were major determinants, with supplementation showing stronger correlations than diet. In early pregnancy, higher levels of 25(OH)D 3 , total 25(OH)D, 24,25(OH) 2 D 3 , and VMR1 were linked to lower risks of SVN-related outcomes, particularly PTB and SVN; however, several of these inverse associations were attenuated after FDR-based correction for multiple testing. In contrast, elevated 3-epi-25(OH)D 3 and VMR2 remained robustly associated with increased risks of SGA and SVN after correction. In mid-pregnancy, 25(OH)D 3 and total 25(OH)D were inversely associated with SGA, whereas 25(OH)D 2 , 3-epi-25(OH)D 3 , and VMR2 were positively associated with PTB, SGA or SVN. Nonlinear patterns included an S-shaped association between 3-epi-25(OH)D 3 and SGA and inverted U-shaped associations between 25(OH)D 2 and multiple outcomes ( P nonlinearity < 0.05). In late pregnancy, associations were largely attenuated, with VMR2 showing only marginal inverse trends for SGA ( P for trend = 0.058) and SVN ( P for trend = 0.064). Trimester-specific vitamin D metabolic profiles showed differential associations with neonatal risks, with stronger corrected evidence for adverse associations related to epimerization-derived markers than for early inverse associations of conventional 25(OH)D metabolites. These findings support comprehensive vitamin D metabolic monitoring and further investigation of tailored supplementation strategies to improve maternal-fetal health. ChiCTR1800016908.