Vitamin D researcher Michael Holick, MD, PhD, and colleagues from the Boston Medical Center report that women in their study who were severely vitamin D deficient during childbirth were about four times more likely to deliver by cesarean section as women with higher vitamin D levels.
In a separate analysis, the research team found that a little more than one in three women (36%) were vitamin D deficient and slightly less than one in four (23%) were severely deficient when they gave birth.
"We are just beginning to recognize that a large percentage of pregnant women are vitamin D deficient and that being on a prenatal vitamin is totally inadequate to bring levels up to where they need to be," Holick tells WebMD.
Holick believes that along with a prenatal vitamin, which typically contains about 400 international units (IU) of vitamin D, pregnant women should take an additional 1,000 IU of the vitamin.
Not So Fast, Expert Says
But maternal-fetal medicine specialist Carl P. Weiner, MD, says more research is needed before such a recommendation would be justified. Weiner is chairman of obstetrics and gynecology at the University of Kansas Medical Center.
"This is an interesting study, but very preliminary, and it should not be seen as the basis for a change in clinical practice," Weiner tells WebMD. "We really can't say if there is a downside or an upside to additional vitamin D."
Just 43 (17%) of the 253 women in the study had cesarean deliveries.
Holick, study co-author Anne Merewood, MPH, and colleagues accessed maternal and infant vitamin D levels, as measured by serum 25-hydroxyvitamin D.
Although there is no consensus on optimal levels of serum vitamin D, Holick says a serum 25-hydroxyvitamin D level of 15 nanograms (ng) per milliliter or below indicates severe vitamin D insufficiency.
A total of 28% of the women in the study had serum vitamin D levels of 15 ng per milliliter or below.
After controlling for maternal risk factors for cesarean delivery, including age, race, and insurance status, the researchers found that women with severe vitamin D deficiencies were four times more likely to have cesarean deliveries as women with higher serum vitamin D levels.
The research appears in the latest online issue of the Journal of Clinical Endocrinology & Metabolism.
Vitamin D and C-section
Merewood tells WebMD that this might explain why women with insufficient vitamin D stores would be more likely to require C-sections.
"That is really just a theory at this point," she says. "We definitely haven't identified a causal pathway."
"We know that some women are more at risk for vitamin D deficiency than others, including women with dark skin, those living in northern climates, and women who are veiled," she says.
Weiner says the study's small size and the fact that the researchers did not control for important fetus-related causes for C-sections limit the ability to draw conclusions from the research.
He points out that vitamin D levels tend to rise and fall with the seasons because exposure to sunlight is the most efficient way to boost levels of the vitamin.
"If this were a strong association you would expect to see cesarean section rates go down in the summer and up in the winter, but they are not particularly seasonal," he says.
He adds, however, that a larger study examining a possible link between vitamin D levels and C-section rates may be justified.
"If vitamin D could improve pregnancy outcomes it is in society's best interest to find this out," he says.