Comparison study of the correlation between free and total 25(OH)D in maternal and umbilical blood and early-life physical development parameters.

Objectives: To investigate the association between free and total 25(OH)D levels and fetal growth during pregnancy, and to evaluate whether free 25(OH)D is a better indicator of vitamin D status compared to total 25(OH)D.

Methods: A total of 160 pregnant women and their newborns were consecutively recruited from a Jiangsu hospital (March-May 2019). Maternal blood was collected before delivery, and cord blood was collected after umbilical cord disconnection. Serum free and total 25(OH)D were measured by ELISA. Clinical data, including pregnancy records, neonatal assessments, and 6-month follow-ups, were extracted from electronic medical records. Associations between vitamin D levels and developmental outcomes were analyzed using univariate correlation analysis and multivariable linear regression in SPSS 20.0 (p<0.05).

Results: Maternal-umbilical free 25(OH)D correlation was stronger than total (rho=0.699 vs. 0.475; both p<0.001). Maternal free 25(OH)D showed stronger associations with early pregnancy femur length (11.37 ± 3.39 mm; β=2.053), late pregnancy biparietal diameter (82.65 ± 4.60 mm; β=-0.273), and femur length growth efficiency (0.89 ± 0.15 mm/week; β=-0.289) than total 25(OH)D (all p<0.05). Umbilical free 25(OH)D correlated with mid-pregnancy transverse cerebellar diameter (22.0[21.0-23.0] mm; β=-0.231) and femur length growth efficiency (0.92[0.78-1.05] mm/week; β=-0.224) (p<0.05 and p<0.01). Maternal free 25(OH)D negatively associated with late pregnancy head circumference (330.11 ± 7.89 mm; β=-9.050, p<0.01).

Conclusions: Compared to total 25(OH)D, free 25(OH)D in maternal and umbilical blood may be more strongly associated with early-life physical growth parameters.