Researchers say they have found evidence that Vitamin D supplements for pregnant women in the world's colder, darker countries may stave off multiple sclerosis (MS) in their offspring.
The finding adds to a growing body of work showing a link between low Vitamin D levels and the debilitating disease, which sees the immune system attacking the body's own nerve fibres.
Data on more than 150,000 MS patients born in places north of 52 degrees revealed a heightened risk for those born in April - a month preceded by a long period without sunlight, said a paper in the Journal of Neurology, Neurosurgery and Psychiatry.
Of the total, 13,300 were born in April compared to 11,600 in November - a lower-risk month after summer in the northern hemisphere, British researchers wrote.
"Month of birth has a significant effect on subsequent MS risk," they said.
"This is likely to be due to ultraviolet light exposure and maternal Vitamin D levels."
They added, though, it could also be "any factor that varies in a similar seasonal and latitudinal manner".
The data was taken from individuals born between 1930 and 1980, from studies done in Britain, the United States, Italy, Israel, Finland, Scotland, Sweden and Canada -- parts of which see little sunlight between the months of October and March.
About 100,000 people in Britain and about 400,000 in the United States are believed to suffer from MS, a disease that affects vision, movement, balance, sensation, bladder control and eventually also memory and thinking. There is no cure.
Suspected links between a lack of vitamin D and an increased risk of death, including from heart disease and certain types of cancer, have been the subject of medical research for several years.
Researchers have also focused on its possible role in MS.
"It is thought that maternal Vitamin D levels during pregnancy affect the immune status of the developing foetus, and hence modulates subsequent MS risk," wrote the authors.
North of 52 degrees latitude lies the northern parts of England, the Scandinavian countries, and most of Russia and Canada.
No studies from the southern hemisphere were included in the analysis.