Birth Month Tied to Multiple Sclerosis Risk

The season in which you’re born may affect your risk of developing multiple sclerosis (search), a new study suggests.

Researchers found that babies born in May appear to have the highest risk of developing the brain nervous system disorder in adulthood, and those born in November have the lowest risk.

They say these findings support the idea that a combination of genetics and environmental factors before and after birth may affect the development of the brain and nervous system, therefore affecting the risk of multiple sclerosis in adulthood.

Birth Month May Affect Multiple Sclerosis Risk

In the study, which appears in the Dec. 7 online edition of the British Medical Journal, researchers looked at the association between birth month and multiple sclerosis risk among 17,874 Canadian and 11,502 British adults with multiple sclerosis and compared it with a similar group of healthy adults from the general population as well as their unaffected brothers and sisters.

The study showed that in Canada, significantly fewer people with multiple sclerosis were born in November. The same held true in Britain, and the number of babies born with MS in December was also lower.

Combining those results with a database from Denmark and Sweden, researchers found that there was an overall 13 percent increase in multiple sclerosis risk for people born in May compared with November and a 19 percent decreased risk for those born in November compared with May.

They say the seasonal effect appeared strongest in Scotland, where the prevalence of multiple sclerosis is highest.

Researchers say previous studies have suggested that exposure to sun or seasonal variations in vitamin D exposure during pregnancy — by affecting the brain development of the fetus — may explain the seasonal differences in multiple sclerosis risk. But more research is needed to explain the link between month of birth and multiple sclerosis risk in adulthood.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Willer, C. British Medical Journal, online first edition, Dec. 7, 2004. News release, British Medical Journal.