Mike: Hello, I'm Mike Clarke, podcast editor for the Cochrane Library. The hundreds of reviews from the Cochrane Pregnancy and Childbirth Group include several of vitamin supplementation during pregnancy. One of these looks at Vitamin D and its third update was published in July 2024. We asked one of the authors, Lia Kostiuk, a board-certified Preventive Medicine physician in the USA to tell us about the latest findings in this podcast.

Lia: Many people are deficient in vitamin D, even in countries with sun exposure all year round. This might be a particular problem during pregnancy and our Cochrane review includes clinical trials of vitamin D supplementation, alone or in combination with calcium or other vitamins and minerals, during pregnancy, compared to placebo or no intervention. We assessed the trustworthiness of each study and compared and summarised their results and quality.
The previous version of this review, published in 2019, included 30 studies but, following our detailed assessment of their trustworthiness, we have excluded one of these because it has been retracted and moved 20 studies to 'awaiting assessment' because their reports did not pass the trustworthiness assessment. We also added one new study, so that this 2024 version includes a total of 10 studies, with 34 other studies awaiting assessment and seven ongoing studies.
This revised evidence base has led to changes to our conclusions. Firstly, eight studies, with just over 2300 women, assessed the effects of vitamin D supplementation compared to no intervention or a placebo on outcomes such as hypertension, gestational diabetes, preterm birth, and kidney disease. Overall, this evidence remains inconclusive. Although there are some indications of potential benefits, such as a possible reduction in severe postpartum bleeding - based on a single study - and a potential decrease in the risk of low birth weight, it is crucial to note that the current evidence cannot entirely rule out that these increase with Vitamin D supplementation.
The effect of vitamin D combined with calcium was evaluated in one study with 84 participants. Not surprisingly, given its small size, this study did not provide conclusive results on whether this combination prevents preterm birth or low birth weight, compared to no intervention of placebo.
The tenth study tested vitamin D combined with calcium and other vitamins and minerals compared to calcium and other vitamins and minerals, without vitamin D. This involved almost 1300 women but does not offer clear evidence on whether the addition of vitamin D prevents gestational diabetes, maternal adverse events, preterm birth, or low birth weight.
In summary, while there is some suggestive evidence regarding the potential benefits of vitamin D supplementation during pregnancy, particularly for severe postpartum bleeding and possibly low birth weight, the current body of evidence does not provide a definitive answer. This means that further research is needed to better understand the role of vitamin D and its combinations with other supplements in improving pregnancy outcomes.

Mike: If you would like to read this latest version of the review and watch for its next update, just go to CochraneLibrary.com and search ‘vitamin D in pregnancy’ to find it.