Context: Current approaches to antenatal vitamin D supplementation do not account for interindividual differences in 25-hydroxyvitamin D (25(OH)D) response.
Objective: We assessed which maternal and environmental characteristics were associated with 25(OH)D after supplementation with cholecalciferol.
Design: Within-randomization-group analysis of participants in the Maternal Vitamin D Osteoporosis Study trial of vitamin D supplementation in pregnancy.
Setting: Hospital antenatal clinics.
Participants: A total of 829 pregnant women (422 placebo, 407 cholecalciferol). At 14 and 34 weeks of gestation, maternal anthropometry, health, and lifestyle were assessed and 25(OH)D measured. Compliance was determined using pill counts at 19 and 34 weeks.
Interventions: 1000 IU/d of cholecalciferol or matched placebo from 14 weeks of gestation until delivery.
Main outcome measure: 25(OH)D at 34 weeks, measured in a single batch (Diasorin Liaison).
Results: 25(OH)D at 34 weeks of gestation was higher in the women randomized to vitamin D (mean [SD], 67.7 [21.3] nmol/L) compared with placebo (43.1 [22.5] nmol/L; P < .001). In women randomized to cholecalciferol, higher pregnancy weight gain from 14 to 34 weeks of gestation (kg) (β = -0.81 [95% confidence interval -1.39, -0.22]), lower compliance with study medication (%) (β = -0.28 [-0.072, -0.48]), lower early pregnancy 25(OH)D (nmol/L) (β = 0.28 [0.16, 0.40]), and delivery in the winter vs the summer (β = -10.5 [-6.4, -14.6]) were independently associated with lower 25(OH)D at 34 weeks of gestation.
Conclusions: Women who gained more weight during pregnancy had lower 25(OH)D in early pregnancy and delivered in winter achieved a lower 25(OH)D in late pregnancy when supplemented with 1000 IU/d cholecalciferol. Future studies should aim to determine appropriate doses to enable consistent repletion of 25(OH)D during pregnancy.