Vitamin D and pregnancy: Why mother and baby need it – and why women should not supplement on their own

Ever heard of a Vitamin D deficiency? Pregnant women in Singapore are lacking in this micronutrient, which is important for bone health and supporting the immune system, and has a role to play in the health of a mother and her unborn baby.

This finding was revealed in a multi-country study involving 1,700 women between 18 and 38 years old in Singapore, New Zealand and the United Kingdom (UK).

Researchers found that nine out of 10 women in these countries had marginal or low levels of Vitamin D (as well as B vitamins such as folate, riboflavin and Vitamin B1), around the time of conception.

The study was a collaboration between universities in the three countries (Yong Loo Lin School of Medicine at the National University of Singapore, the University of Auckland and the University of Southampton), as well as Singapore’s National University Hospital, and the Agency for Science, Technology and Research, and the UK’s National Institute of Health and Care Research (NIHR) Biomedical Research Centre.

The World Health Organization says it’s common for pregnant women to be deficient in Vitamin A, iron and folate due to the mother and baby’s increased nutrient requirements.

However, Vitamin D deficiency among pregnant women in Singapore might come as a surprise. Vitamin D is produced in the skin upon exposure to the sun – and sunshine is something Singapore has plenty of.

WHY SINGAPORE WOMEN ARE DEFICIENT IN VITAMIN D  

Jennifer Shim, a senior dietitian at Parkway East Hospital, said people should get at least five to 30 minutes of sun exposure, at least twice a week. The Singhealth website advised letting the sun shine on your arms and legs, between 10am and 3pm.

The reason we’re not getting at least this amount of sun exposure is our sun-protection behaviour. “Using sunscreen, umbrellas, staying indoors or seeking shade, as well as wearing long sleeves, can block the source of Vitamin D production from sunlight,” she said.

A 2019 study on sun exposure and sun safety habits among Singapore adults found that sun protection awareness in women was higher than men’s, Shim added. 

Professor Yong Eu Leong of the NUS’ Yong Loo Lin School of Medicine said women are also generally more Vitamin D-deficient as they tend to remain indoors.

Younger women, in particular, “are not exposed to sunlight during working hours”, he said, and so are missing out on sun exposure.

Other factors also affect a person’s Vitamin D levels, said Shim. These include being post-menopausal and above 70 years old, having poor kidney function, being obese, and having medical conditions such as lymphoma or parathyroid disease (where the parathyroid gland becomes enlarged).

VITAMIN D AND GETTING PREGNANT

Vitamin D influences the process in which the ovaries produce eggs and release them for fertilisation. Dr Han How Chuan, obstetrician and gynaecologist at Mount Elizabeth Hospital, said that studies on Vitamin D and assisted reproductive treatments have found that being deficient in the micronutrient can be a risk marker for reduced fertility.  

Women undergoing assisted reproductive treatments who have sufficient levels of Vitamin D have a “higher live birth rate compared with those who are deficient or insufficient in Vitamin D”, he added.

However, routine screening for Vitamin D deficiency, whether in pregnancy or for those trying to get pregnant, is not the norm. “Studies have shown a high prevalence of pregnant women who are Vitamin D deficient; many of these women still go on to have healthy pregnancies,” said Dr Han.

The role Vitamin D plays in increasing the chances of getting pregnant is also not conclusive. Associate Professor Chan Shiao-Yng, from the NUS Yong Loo Lin School of Medicine, said Vitamin D is not part of routine fertility care as there is “limited evidence” that Vitamin D supplementation “improves fertility”.

VITAMIN D FOR A HEALTHY PREGNANCY AND BABY 

Being deficient in Vitamin D has consequences for the mother and baby.

Both Assoc Prof Chan and Dr Han said a lack of Vitamin D has been associated with an increased risk of miscarriage, observed in a 2024 study on the influence of Vitamin D and pregnancy outcomes.

It can also result in anaemia during pregnancy and postpartum depression, said Dr Han. 

Assoc Prof Chan added a Vitamin D deficiency can lead to a higher risk of preeclampsia (high blood pressure in pregnancy), delivering pre-term (before 37 weeks), gestational diabetes, having a caesarean section, and even postpartum haemorrhage (heavy bleeding after giving birth).

“Vitamin D is important for bone health, supporting our immune system as well as regulating the amount of calcium and phosphate in the body,” said Dr Han. Not having enough Vitamin D in pregnancy can adversely affect the foetus’ bone health and development.

Low Vitamin D levels in the mother also means low Vitamin D levels in the foetus and in the woman’s breast milk, said Assoc Prof Chan. This could increase the risk of “children having lower bone mineral density, and higher risk of asthma, learning difficulties and autism”, she said.

SHOULD YOU THEN ASK YOUR DOCTOR FOR VITAMIN D SUPPLEMENTS?

Pregnant women already get Vitamin D, together with folic acid, calcium and iron, from the prenatal vitamins their doctor prescribes – typically 400 IU of Vitamin D per tablet, said Dr Han. This amount is the Singapore Health Promotion Board’s (HPB) recommended daily allowance for pregnant and lactating women.

The clinical data doesn’t support prescribing Vitamin D as an add-on supplement for mums-to-be – unless a deficiency is suspected, said Assoc Prof Chan. 

“For folic acid, clinical trials have provided strong evidence that supplementation reduces foetal anomalies and improves birth outcomes. However, evidence for a causative effect of Vitamin D is not as strong,” she added.

Findings in clinical trials have been inconsistent, she said, “with some showing benefits on different outcomes while others showing no benefit for those same outcomes.

“Given the present uncertainties, routine widespread supplementation [of Vitamin D] to every woman has not been recommended by professional bodies.”

Dr Han advised speaking with your obstetrician “to assess your risk of Vitamin D deficiency and to consider having your blood level of Vitamin D checked before or during pregnancy”.

HOW MUCH VITAMIN D IS ENOUGH? 

HPB advises that women who are pregnant or breastfeeding receive 10mcg or 400 IU of Vitamin D a day; the recommended dietary allowance for non-pregnant women and men is 2.5mcg or 100 IU a day.

These amounts are based on a person living in Singapore with “its tropical climate and consistent, strong sunlight exposure all-year round”, said Shim.

For pregnant women identified as Vitamin D-deficient, Dr Han said the American College of Obstetricians and Gynecologists recommends that 1,000-2,000 IU (or 25–50mcg) per day is “considered safe”.

It may not be obvious that you are deficient in Vitamin D. “Some signs and symptoms include fatigue, bone pain, muscle weakness and mood changes, such as depression,” said Shim. “However, some people may experience no signs or symptoms.”

Aside from sun exposure and Vitamin D supplements, you can also get your daily dose of Vitamin D from food. These include: Oily fish like salmon, mackerel and sardines, eggs, liver, and fortified foods such as milk, soya milk, cereals, yoghurt, orange juice and margarine

Do remember that too much Vitamin D does you no good as well.  “While Vitamin D is generally beneficial, excessive intake can lead to potential side effects such as loss of appetite, weak bones, weak muscles and kidney stones,” said Shim.