Vitamin D supplements in pregnancy: what’s the latest evidence?

In this blog for pregnant women, Emily Carter, Cochrane UK Fellow and Obstetrics and Gynaecology Registrar, looks at the latest Cochrane evidence on Vitamin D supplementation in pregnancy and how it may help reduce risks for mums and babies in the UK. This blog is part of a series called ‘Maternity Matters’, you can read the rest here.

What we already know

Vitamin D forms from sunlight on the skin and is used by the body to absorb calcium, needed for healthy bones and teeth. Up to half of all pregnant women in the UK are affected by vitamin D deficiency, which is more common in winter than in summer. Vitamin D deficiency disproportionately affects black and minority ethnic groups, those who cover their skin, those who are overweight and those with chronic illness, and particular care should be taken to advise these women (National Institute for Health and Care Excellence, 2019).

Pregnant women who are deficient in vitamin D (blood serum level <50nm) are more likely to have pregnancy complications including pre-eclampsia, diabetes, preterm birth and small babies. Babies born with vitamin D deficiency may have affected bone growth or in severe cases rickets (flexible bones). Vitamin D deficiency has also been linked with the development of childhood allergy (European Food Safety Authority, 2016; Royal College of Obstetricians & Gynaecologists, 2014).

An evidence gap

Whilst the Royal College of Obstetricians and Gynaecologists (RCOG) supports supplementing pregnant women with vitamin D (Royal College of Obstetricians & Gynaecologists, 2014), until now there has been a lack of evidence confirming the benefits and harms of this. In practice, it has been confusing what to advise women – should they be given a low dose supplement, a high-dose supplement, or should they be tested for vitamin D deficiency and treated if deficient in this important nutrient?

Although most pregnancy multivitamins contain 10 micrograms (400 international units), for women at high risk of vitamin D deficiency this may not be enough and they might benefit from a higher dose supplement. The criteria for who precisely who should be considered high risk is unclear. In practice, local guidelines have used ‘common sense’ criteria, recommending a higher dose in women with some chronic illnesses and those who are less likely to absorb vitamin D from sunlight on the skin; but this has not been convincingly evidence-based so far (NHS Oxfordshire Clinical Commissioning Group, 2017; Royal College of Obstetricians & Gynaecologists, 2014).

It is also not clear what dose is required to gain the benefit of supplementation. There is also the issue of potential harms – we need to know what the best available evidence is about the possibility of harm from treatment in order to counsel women about the safety of taking pregnancy supplements.

What does the new Cochrane evidence add?

An important new Cochrane review update (Palacios et al., 2019) summarises the evidence base for Vitamin D supplementation in pregnancy; it includes 30 research studies and over 3700 pregnant women were included. Before this review, we knew that babies from mothers who lacked vitamin D have poorer outcomes, but it had not been convincingly demonstrated that supplementation improved outcomes for those at risk.

It showed that taking vitamin D supplements in pregnancy:

  • Probably reduces the risk of getting pre-eclampsia and gestational diabetes
  • May reduce the risk of having a low-birthweight baby.
  • May reduce the risk of severe bleeding after birth.
  • May make no difference to the risk of preterm birth before 37 weeks

Only one of the studies looked at whether there was any harm from taking vitamin D and nothing certain was shown.

Taking vitamin D and calcium supplements together in pregnancy: what does the evidence say?

 The review also shows that women who take vitamin D and calcium together in pregnancy probably have a reduced risk of developing pre-eclampsia. However, there may be an increased risk of preterm birth less than 37 weeks. These results warrant further research.

Whilst there are potential harms of taking combined calcium and vitamin D supplementation, the benefits for those at risk of pre-eclampsia may outweigh these harms. Women considering this should discuss with their midwife and obstetrician in early pregnancy.

What questions do we still have left to answer?

Should all women be screened for vitamin D deficiency?

There is no evidence supporting the use of universal screening for vitamin D deficiency in pregnancy, or telling us which groups of women this might be most useful for. The Royal College of Obstetricians and Gynaecologists recommends screening for very high-risk women, for instance those with symptoms, brittle bones, or those with chronic illness who may not be able to absorb vitamins from the intestines. Many local guidelines follow this advice (NHS Oxfordshire Clinical Commissioning Group, 2017). This is ‘common sense’ practice but it is not convincingly evidence-based.

What dose of vitamin D supplementation is safe in pregnancy?

The effects and safety of different vitamin D supplementation regimens in pregnancy is the topic of a forthcoming Cochrane review.*

Where does this leave you?

Pregnant women may require higher doses of some vitamins, including vitamin D, from conception or early pregnancy. The RCOG recommends supplementing all pregnant women with vitamin D and high-risk women with high dose vitamin D. By booking the pregnancy on time, your community midwife can advise you early on to get the full benefit of treatment.

There is now moderate-certainty evidence that vitamin D supplementation probably reduces common risks associated with pregnancy including pre-eclampsia and gestational diabetes, and low-certainty evidence that supplementation may reduce the risk of having a low-birthweight baby or bleeding after birth.

There is now moderate-certainty evidence that vitamin D and calcium supplementation probably reduces the risk of pre-eclampsia. However, as there is low-quality evidence that this may increase the risk of pre-term delivery, this risk should be discussed with your midwife or obstetrician, as further research is required to clarify this risk.

What can be done to spread the message?

This latest evidence, which now gives greater certainty that vitamin D supplementation is probably beneficial in reducing some common risks associated with pregnancy, may be helpful in informing any planned updates of existing UK and international guidelines and their local implementation by NHS Clinical Commissioning Groups.

Development of patient information leaflets and online information which can be read at home can facilitate discussion with your obstetrician and midwife regarding individualised pregnancy risks and which supplements will be of most benefit.

The development of a clinical calculator to assess each woman at her first antenatal booking appointment for the need to take vitamin D, aspirin, calcium, high-dose folic acid and low molecular weight heparin (blood thinning injections for those at high risk of a blood clot) would help reach those women who may not know they could reduce their pregnancy risk by taking these supplements or treatments early on in pregnancy.

*When the Cochrane Review on vitamin D supplementation regimens in pregnancy is published, this blog will be updated to report on the evidence.

Join in the conversation with @CochraneUK @Emily_Carter88 #MaternityMatters or leave a comment on the blog.

References may be found here.

Emily Carter has nothing to disclose.


Emily Carter

About Emily Carter

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Emily is a ST4 Speciality Registrar in Obstetrics and Gynaecology in the Oxford Deanery. She is passionate about medical education and with the support of Cochrane UK hopes to encourage medical and dental trainees from the UK and Ireland across different regions and specialities to engage with evidence based clinical practice. Emily read medicine at the University of Manchester, graduating in 2013 with a Masters of Research (Distinction) and the Boyd Prize. She was awarded an Academic Foundation Programme in Medical Education and has published and presented internationally since 2012. She gained membership of the Royal College of Obstetricians and Gynaecologists in 2018. She secured the Leo-Pharma scholarship for MRes funding, conference scholarships; BMFMS highest scoring student abstract (2012) and RSM (2015). She holds grants from Health Education England, Friends of the Horton, and Oxford Hospital Charities for clinical research and quality improvement projects. She was the recipient of the BSGE trainees’ bursary and is a Laparoscopic Simulation lead for the Oxford deanery (2018-2019). Her area of specialist interest is urogynaecology and she currently updating the Cochrane systematic review of mini-slings.

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