11:40 AM PST on Tuesday, December 7, 2004

This may be hard to believe, but your birthday may play a role in your risk for developing multiple sclerosis.

Studies of more than 42,000 people in Canada, Britain, Denmark and Sweden found that people born in May, in the northern hemisphere, have a 13 percent greater chance of developing MS.

Having a November birthday apparently decreased the risk by 19 percent.

The effect was similar in all of the countries but most prominent in Scotland, which has the highest rate of MS in the world.

Scientists aren't sure why, but they think it's linked to sunlight exposure and a mother's vitamin D levels, while she's pregnant.

MS occurs when immune system cells attack and destroy the myelin sheath that protects the nerve cells in the brain and spinal cord. The cause of the illness is unclear. Scientists believe it may be due to a combination of genetic, dietary and environmental factors.