There’s convincing evidence that vitamin D can help lower risk for a variety of health conditions
And yet, vitamin D deficiencies are common in many people, including pregnant women.
A Cochrane review published in 2016 analysed 15 randomised, controlled trials, involving 2833 women, to determine whether supplementing with vitamin D alone or in combination with calcium can safely improve maternal and foetal outcomes.
The researchers concluded there is evidence that vitamin D supplementation increases pregnant women’s 25-hydroxyvitamin D levels. And it may reduce the risk of pre-eclampsia, gestational diabetes, low birth weight and preterm births. However, vitamin D and calcium supplementation may increase the risk of preterm birth.
Of the 15 trials involved in the review, nine compared the effects of vitamin D supplementation versus no supplementation or a placebo. Six compared vitamin D and calcium supplementation with no supplementation.
Seven of the vitamin D trials showed that women who take vitamin D supplements, particularly on a daily basis, had higher 25-hydroxyvitamin D levels than women who didn’t take vitamin D or took a placebo. Two trials showed that women who took vitamin D had a lower risk of pre-eclampsia, and two trials showed they had a lower risk of gestational diabetes. Three trials linked D supplementation with a lower risk of preterm birth.
Three trials showed that women who took vitamin D during pregnancy were less likely to have babies with birth weights below 2500g, compared with women who didn’t supplement with the vitamin or took a placebo. And four trials found a positive correlation between D supplementation and infant length and head circumference at birth.
Three trials showed that women who took vitamin D and calcium had a lower risk of pre-eclampsia, but also a higher risk of preterm birth compared to women who didn’t take D and calcium. The supplement duo didn’t have an effect on gestational diabetes or low birth weight.
Despite this evidence, the researchers said it’s unclear whether pregnant women should supplement with vitamin D. 'Currently, the number of high-quality trials with large sample sizes and outcomes reported is too limited to draw definite conclusions on its usefulness and safety,' they wrote.