Vitamin D in Pregnancy May Help Winter Babies' Bones

There has been much talk of the importance of vitamin D supplements for everyone, not least pregnant women. A new study says it's especially vital for winter babies.

Vitamin D in Pregnancy May Help Winter Babies' Bones SpeedKingz/Shutterstock

Everyone knows to take their prenatal vitamins, but there's much conflicting advice on additional supplements during pregnancy. Do any of them actually help, or are you swallowing all those giant pills for nothing? Turns out, one that has been the subject of much debate among doctors, vitamin D, may not be necessary—unless your baby will be born in the winter, according to a new study published in The Lancet Diabetes and Endocrinology journal.

The 'sunshine vitamin' lacking in winter

Vitamin D helps the body absorb calcium and is crucial for bone health, and this is the first random trial looking at the effects of vitamin D supplementation in pregnancy on baby's bones. Researchers had over 1,000 pregnant women take either 1,000 IU of vitamin D or a placebo, and then measured their babies' bone mass after birth. They found that although the supplements raised the level of the women's vitamin D, overall it had no effect on the babies' bone mass.

But upon closer inspection, they saw that there was actually a rise in bone mass among some babies whose moms took vitamin D—those who were born from December to February. "These pregnancies, which see a steep decline in vitamin D status of the mom between summer and winter, have a complete correction of this decline with the 1,000 units of vitamin D daily," study author Cyrus Cooper, a professor of rheumatology at the University of Southampton in the U.K., tells Fit Pregnancy. "Sunlight is the best way of ensuring vitamin D repletion, but in temperate parts of the world, it is insufficient. Clearly, the maintenance of adequate vitamin D status throughout the year is the important finding from our research."

While the levels of vitamin D in the moms of winter babies who took supplements rose, the levels of those who didn't take them dropped. Because baby's bones strengthen during the third trimester, vitamin D supplementation was crucial for the moms who were in their last weeks of pregnancy in the winter. "Supplementation was more important when pregnancies started in summer, as these were the ones where there was the [greatest] decline in winter," Cooper says. "It was in this circumstance that supplementation at the 1,000 unit dose was most effective."

How much should you take?

Normal prenatal vitamins usually only have 400 IU of vitamin D, and The American College of Obstetricians and Gynecologists' (ACOG) official position is not to recommend more until studies show effectiveness—so this research could change things. ACOG does say, though, that doses of 1,000-2,000 IU, and possibly even up to 4,000, are safe among pregnant women at risk for vitamin D deficiency (such as women in colder climates or women with darker skin). Although some studies have suggested lack of vitamin D is linked with other pregnancy problems like preeclampsia and preterm labor, ACOG says there's not enough evidence to recommend supplementation to prevent them.

But it's likely vitamin D does have other benefits. "From observational studies, it is possible that babies from supplemented moms will have higher muscle mass, lower fat mass and lower risk of asthma and allergy," Cooper says. He says to follow the 400 IU guidance, but given the results of this study, "those women entering autumn and winter while expecting think about using a higher dose."

Although the study did not focus on this, Vitamin D is also important for babies after birth, likely even more so if they are born in the winter. The American Academy of Pediatrics recommends 400 IU of vitamin D per day for infants. Breastfed babies generally do not meet this requirement, but even formula fortified with vitamin D might not be sufficient. So all babies should take liquid supplements to make sure they're getting enough.

Believe it or not, an infant's bone health from the day he is born—and even before—can affect his risk of the bone disease osteoporosis and fractures as an older adult. Further research is needed, but this study shows that vitamin D supplementation in pregnancy does work to improve winter babies' bone health.

Related: Vitamin D May Make Childbirth Less Painful