John: Hello, I'm John Hilton, editor of the Cochrane Editorial unit. Many pregnant women have Vitamin D deficiency, and might consider taking oral supplements to counter this. In January 2016, Cristina Palacios from the University of Puerto Rico and colleagues updated their Cochrane Review of the evidence. She tells us more in this podcast.

Cristina: Vitamin D deficiency or insufficiency is common among pregnant women worldwide. Vitamin D supplementation has been shown to improve maternal vitamin D status and some studies have suggested that it could also protect against adverse pregnancy outcomes. Our Cochrane Review examines whether oral supplements with vitamin D alone or in combination with calcium can safely improve maternal and neonatal outcomes when taken by women during pregnancy. It suggests that there might be some benefits, but we need to remain cautious because the evidence remains relatively weak.
The initial version of the review was published in 2011 and the update now includes 15 trials, with a total of nearly 3000 women. Nine trials compared vitamin D versus no supplementation or a placebo, and the other six compared the combination of vitamin D and calcium with no supplementation.
Data from seven trials involving 868 women show that women who received vitamin D supplements alone had higher 25-hydroxyvitamin D serum levels than those receiving no intervention or placebo. Also, data from two trials involving approximately 200 women suggest that those who received vitamin D supplements may be less likely to develop pre-eclampsia than those receiving no intervention or placebo. With respect to the babies, data from three trials involving nearly 500 pregnant women suggest that vitamin D supplementation reduces the risk of preterm birth compared to no intervention or placebo, and babies weighing under 2500 g were also less common for the supplemented women than for those receiving no intervention or placebo. Women who received vitamin D with calcium had a 50% lower risk of pre-eclampsia than those not receiving any intervention but a 50% increased risk of preterm birth.
In conclusion, the new studies have provided more evidence on the effects of the use of Vitamin D supplements, with and without calcium, during pregnancy. Supplementing pregnant women with vitamin D increases serum 25-hydroxyvitamin D levels at term and may reduce the risk of pre-eclampsia, low birthweight and preterm birth. However, when vitamin D and calcium are combined, the risk of preterm birth is increased. Data on adverse effects were lacking in all studies. The clinical significance of the increased serum 25-hydroxyvitamin D concentrations is still unclear and, so, our results need to be interpreted with caution. It is still unclear whether vitamin D supplementation should be given as part of routine antenatal care to all women to improve maternal and infant outcomes and further rigorous randomised trials are required to resolve this uncertainty.

John: If you would like to explore the current evidence in more detail, and watch for further updates of this review as information from ongoing and future studies becomes available, you can find the review with a search for 'Vitamin D and pregnancy' at Cochrane Library dot com.