The study, which is the largest to date to look at the association between vitamin D and preterm birth, suggests that African-American and Puerto Rican women who have low vitamin D status are more likely to have a pre-term baby than those with higher levels.
Writing in the American Journal of Epidemiology, researchers from the University of Pittsburgh Graduate School of Public Health reported that the incidence of spontaneous, preterm birth decreased by as much as 30% when vitamin D levels in the blood increased.
Pre-term birth was defined as naturally going into labour two or more weeks before the 37 weeks of pregnancy considered full-term.
"Vitamin D is unique in that while we get it from our diets, our primary source is our body making it from sunlight," said lead author Dr Lisa Bodnar.
Dr. Bodnar and her co-authors, whose work was funded by the US National Institutes of Health, did not find a similar relationship between maternal vitamin D levels and preterm birth in white women.
"We were concerned that finding this association only in non-white women meant that other factors we did not measure accounted for the link between low vitamin D levels and spontaneous preterm birth in black and Puerto Rican mothers," said Bodnar.
However, the team double checked and accounted for the expected influence of discrimination and socioeconomic position, in addition to fish intake and physical activity.
"Even after applying these methods, vitamin D deficiency remained associated with spontaneous preterm birth," she said.
Study details
The researchers used a sample of over 700 cases of preterm birth and 2,600 full-term births collected by the Collaborative Perinatal Project, which was conducted in 12 U.S. medical centres from 1959 to 1965. Blood samples collected by the project were well-preserved and able to be tested for vitamin D levels 40 years later.
The tem noted that novel part of the study was the availability of information from placental examinations - they found that vitamin D deficiency was most strongly related to preterm births with damage to the placenta caused by inflammation.
"This finding may give us insight into the biology connecting low vitamin D and preterm birth," Dr. Simhan said. "It holds great promise and will motivate significant preterm birth research."
"It is critical to repeat this study in a modern sample," added Bodnar - who noted that pregnant women today smoke less, have less sun-exposure and receive more vitamin D in their foods than the mid-century cohort.
"Further, it is especially important to understand how vitamin D influences preterm birth among black mothers. Vitamin D supplementation could be an easy way to reduce the high rates of preterm birth in this group."