Pregnant women who gain more weight and those who deliver in winter may need a higher dose of vitamin D to achieve repletion, new findings published in the Journal of Clinical Endocrinology and Metabolism suggest.

“Vitamin D definitely needs careful consideration in pregnancy,” Cyrus Cooper of the University of Southampton in the UK, one of the new study’s authors, said. The findings were published online October 27.

Vitamin D deficiency in pregnancy is common and linked to poor maternal and foetal outcomes, Cooper and his team write. Currently, the Institute of Medicine recommends that vitamin D insufficiency (25(OH)D less than 50 nmol/L) be avoided in pregnancy, they add.

Vitamin D deficiency in pregnancy is common and linked to poor maternal and foetal outcomes

In non-pregnant individuals, adiposity, age and baseline 25 (OH)D help determine how much vitamin D supplementation increases 25(OH)D levels, the researchers state.

Hemodilution, body composition changes and metabolic factors occurring during pregnancy may also influence vitamin D supplementation response, they add, and understanding these factors could help clinicians individualise supplementation during pregnancy to ensure the best response without toxicity.

To investigate, Cooper and his team performed a within-randomisation-group analysis of 829 pregnant women participating in the Maternal Vitamin D Osteoporosis Study, 422 of whom received placebo and 407 who took 1,000 IU of cholecalciferol from 14 weeks until delivery.

At 34 weeks, 25(OH)D was 67.7 nmol/L in study participants randomised to vitamin D, and 43.1 nmol/L for the placebo group. Among women who received cholecalciferol, higher weight gain from 14 to 34 weeks, lower study medication compliance, lower 25(OH)D levels at baseline, and winter rather than summer delivery were each independently associated with lower 25(OH)D at 34 weeks.

“Vitamin supplementation should be given at 1,000 IU daily to most women in early pregnancy, and especially if winter birth is expected,” Cooper said.

He and his colleagues conclude: “Future studies should aim to determine appropriate doses to enable consistent repletion of 25(OH)D during pregnancy.”

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