Do low vitamin D levels matter during pregnancy?

Many women have low blood levels of vitamin D early in pregnancy, but whether that's a problem for their developing babies is uncertain, researchers have found.

The new study, by scientists in Australia, suggests that vitamin D deficiency among pregnant women may even be the norm, although defining deficiency is tricky.

However, the researchers did not find any solid evidence that it stunts fetal growth or causes other pregnancy-related complications.

The researchers analyzed 18 previous studies in which vitamin D levels were assessed in women in their first 3 months of pregnancy. Of those, five studies included information about the outcome of the pregnancy, including low birth weight and preeclampsia, a maternal complication that can be life-threatening.

The researchers, who reported their findings April 8 in the American Journal of Obstetrics and Gynecology, found no clear definition of vitamin D deficiency during early pregnancy. Rather, women in the previous studies were considered "deficient" if they fell below a certain level based the particular population of women assessed.

Definitions of vitamin D deficiency in the studies varied widely, according to the researchers, from 12.5 to 50 nanomoles (nmol) per liter. Insufficiency ranged between 37.5 and 80 nmol per liter, while sufficiency ranged from above 50 to above 80 nmol per liter.

For comparison, a 2010 Institute of Medicine report states that levels of vitamin D above 50 nmol per liter are necessary for proper bone growth in nearly all people.

"Despite the variation in cut-points applied by studies," the authors wrote, "all reported a significant proportion of their sample to be vitamin D deficient. This has serious implications for the management of pregnancy as many pregnancy care providers are considering or implementing vitamin D supplementation in pregnancy."

Indeed, the only consistent picture to emerge from the analysis was that most women have low levels of vitamin D during pregnancy. White women typically had about twice the blood levels of vitamin D as non-white women, a reflection of the fact that darker skin pigments block ultraviolet rays.

Similarly, women living in northern latitudes had lower levels of vitamin D than those living nearer to the equator, where sunlight is more intense.

If vitamin D deficiency were linked to trouble with pregnancy or fetal development, identifying the problem early on could allow women to take supplements of the nutrient and correct the deficit. To date, no conclusive randomized, controlled study of the effects of vitamin D on pregnancy has been conducted.

But whether pregnant women are getting too little, or perhaps too much, vitamin D has been controversial. In November, the Institute of Medicine issued guidelines calling for pregnant women to receive 600 international units (IUs) of vitamin D daily, with a maximum dose of no more than 4,000 IUs.