Monaz: Hello, I'm Monaz Mehta, editor in the Cochrane Editorial and Methods department. Many pregnant women have Vitamin D deficiency, and might consider taking oral supplements to counter this. In July 2019, Cristina Palacios from Florida International University in the USA, 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, which was first published in 2011, examines whether oral supplements with vitamin D alone or in combination with calcium can safely improve outcomes for the mother and her baby when she takes them during pregnancy. It suggests that there might be some benefits. This information will be used by the World Health Organisation to update their guidelines on vitamin D supplementation during pregnancy.
When the initial version of the review was published in 2011, it included only 6 trials with just over 1000 women. By the 2016 update this had increased to 15 trials with 2800 women. Now, in this third update, we have jumped to 30 trials, with a total of about 7000 women. Twenty-two trials compared vitamin D versus no supplementation or a placebo, and nine trials compared the combination of vitamin D and calcium with no supplementation; but we were not able to use all of the trials in all our analyses. 
Data from four trials involving almost 500 women showed that those who received vitamin D supplements alone during pregnancy probably had a reduction in the risk of pre-eclampsia. Also, those who received vitamin D supplements alone during pregnancy probably had a reduction in the risk of gestational diabetes and having low birthweight baby, which is defined as a baby weighing less than 2500g, compared to women who received placebo or no intervention. However, based on 7 trials in 1600 women, vitamin D supplementation may have little or no effect on the likelihood of a preterm baby being born at less than 37 weeks into the pregnancy. 
When Vitamin D supplementation is combined with calcium, this probably halves the risk of pre-eclampsia but it might increase the risk of preterm birth by 50% compared to no supplementation.
In conclusion, supplementing pregnant women with vitamin D alone probably reduces the risk of a women developing pre-eclampsia or gestational diabetes, or having a low birthweight baby. When calcium is also added, this further reduces the risk of pre-eclampsia but may increase the risk of preterm births and these findings need to be studied further in new research. Additional research is also needed to evaluate maternal adverse events.

Monaz: If you would like to explore this growing evidence base in more detail, and watch for the fourth update, you can find it with a simple search for 'Vitamin D and pregnancy' on Cochrane Library dot com.