The study by researchers at the University of Pittsburgh was able to examine a database of 44,500 women, picking out 717 that had developed pre-eclampsia. Severe pre-eclampsia sometimes requires induced labour and delivery.
Funded by the National Institutes of Health, the study will be published next month in the journal Epidemiology.
The women were part of the Collaborative Perinatal Project that ran from 1959 to 1965, the nation's largest ever study of pregnant women. The women's blood was well preserved enough to be tested for vitamin D levels.
Researchers looked at vitamin D levels prior to 26 weeks gestation and examined whether there was any connection between low levels and pre-eclampsia, a pregnancy disorder signified by high blood pressure and elevated protein levels in urine. Complications of untreated pre-eclampsia can be dangerous, even fatal, to a woman and her baby.
While the researchers did not find a connection between vitamin D and mild pre-eclampsia, it did find a significant correlation between vitamin D and severe pre-eclampsia.
"Vitamin D status of the mothers was not related to pre-eclampsia when we looked at it overall, but when we separated it into two different subtypes, that's where we found a relationship," said Lisa Bodnar, an associate professor of epidemiology at Pitt's Graduate School of Public Health and lead author of the study.
The study does not necessarily suggest, however, that women can prevent pre-eclampsia by taking more vitamin D. It's possible that low vitamin D levels could be a side effect of the pre-eclampsia, which may actually start long before it shows up in a women's blood pressure or urine protein levels.
It could also be that the relationship in the data set, collected in the 1960s, no longer holds true today.
"Yes, we found this relationship, but we don't believe that women should run out and start taking more vitamin D supplements when they are planning a pregnancy or when they're pregnant," said Prof Bodnar. "There's still a lot more research to be done."
Because severe pre-eclampsia is quite rare - occurring in roughly five out of 1000 pregnancies - a very large sample would be needed to examine further the vitamin D link.
"It would take a lot of resources and a lot of people to answer the question right," said Prof Bodnar.
Studies such as this one are hard to do because the data set required is so large, said Mark Caine, director of labour and delivery at West Penn Hospital, calling the Pitt research "a study that nobody else has really done."
He noted that researchers have been looking for causes of pre-eclampsia for some time, referring to a 10- to 15-year study on calcium that ultimately couldn't find a link to the disorder.
He also noted that vitamin D research is now in vogue, as researchers aim to link it to numerous diseases, both in and out of pregnancy.
"People have been looking at vitamin D levels for several different adverse pregnancy outcomes - intrauterine growth restriction, preterm labour, diabetes," he said. "The studies really still are all over the place."
