Vitamin D in pregnancy - is it any good?

Our current knowledge of the benefits and drawbacks of vitamin D supplements in pregnancy is insufficient to recommend their use, according to researchers at the BMJ. Vitamin D is essential for the healthy development and maintenance of bones, teeth and muscle with numerous studies indicating that it may help fight against heart disease, cancer, respiratory infections and asthma, alongside pregnancy related conditions like preeclampsia and gestational diabetes. However, these studies have a wide range of results, with conflicting recommendations being commonplace among international and national organisations.

This ambiguity prompted a team led by Dr Daniel Roth at The Hospital for Sick Children in Toronto to assess the current and future evidence on vitamin D supplements during pregnancy. They analysed results from 43 randomised controlled trials involving 8,406 women, taking into account differences in study design and execution to minimise bias, however there was still significant variation.

On the surface, the results show that taking vitamin D supplements increased levels in the mother’s, and umbilical cord blood, and babies were on average 58g heavier when born. There is also evidence that vitamin D reduces the risk of wheeze in children by age 3; however on further exploration these data were drawn from only two trials, each with a small sample size. There is also no clear evidence of the benefit on maternal health conditions, birth and safety outcomes. This is a common picture spanning the trials, with an average sample size of 133, and many trials prone to bias. Therefore, the team concluded that there is currently insufficient evidence supporting the use of prenatal vitamin D supplementation.

Looking to the future, the team identified another 35 complete but unpublished, ongoing or planned trials that could add another 12,530 participants to forthcoming reviews. They suggest that within the next decade we will know more about vitamin D in pregnancy, however the lack of a coordinated approach and insufficient funding severely hampers the ability to conduct large trials, impeding the progress towards a deeper understanding of vitamin D in pregnancy.