Perinatal vitamin D metabolism. III. Factors influencing late gestational human serum 25-hydroxyvitamin D.

Sera were obtained during the last trimester from both black and white women receiving prenatal care for determinations of calcium, magnesium, and 25-hydroxyvitamin D (25-OHD). The mean serum 25-OHD concentrations were 15.4 +/- 5.9 ng, per milliliter (S.D.) in February (n = 56) and 42.1 +/- 13.9 ng.per milliliter in August (n = 61). There was no significant racial difference in serum 25-OHD in February or August. No seasonal difference in calcium was noted but magnesium concentrations were lower in August. No correlation existed between serum 25-OHD and serum calcium or magnesium, parity, time from last pregnancy, maternal age, or social class. Concentrations of 25-OHD showed weak correlations with vitamin D intake (r = 0.32) and duration of gestation (r = 0.26) in the winter only. These data suggest that, regardless of race, ultraviolet exposure is the major determinant of maternal serum 25-OHD concentration in St. Louis. Dietary intake and and other factors may be important during periods of decreased ultraviolet exposure.