People born in May in the northern hemisphere have a higher than average risk of developing multiple sclerosis, researchers said on Tuesday.

An analysis of data from studies of more than 42,000 people in Canada, Britain, Denmark and Sweden showed that May babies have a 13 percent increased chance of suffering from the illness later in life, but that having a November birthday decreased the average odds by 19 percent.

“If you are born in May, your risk is higher than any other month and if you are born in November your risk is lower than any other month,” Professor George Ebers, of Radcliffe Infirmary at the University of Oxford, said in an interview.

The effect was similar in all the countries but most prominent in Scotland, which has the highest rate of multiple sclerosis (MS) in the world, according to Ebers.

Although the scientists cannot explain the correlation between birth month and MS, they suspect it could be linked to exposure to sunlight and the mother’s vitamin D levels, which could influence the child’s development.

Shorter days during the winter months in the northern hemisphere limit the amount of sunlight women are exposed to during pregnancy. The body makes vitamin D from sunlight. Foods such as oily fish and egg yolk are rich in the vitamin.

“It looks like something must be happening very early, either in gestation or around the time of birth, that determines one’s subsequent risk of getting MS,” Ebers added.

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.

The illness is rare in Africa and most common in people living in colder countries. Most people are diagnosed with the disorder between the ages of 20-50.

More women than men suffer from MS, which can be difficult to diagnose because symptoms such as tingling, fatigue, loss of balance and slurred speech are intermittent.

Ebers and his colleagues compared birth months of 17,874 patients in Canada and 11,502 in Britain with their unaffected siblings and the general population.

They pooled their findings with data from studies in Denmark and Sweden.

“The risk factors responsible for the effect of timing of birth must vary seasonally and probably interact with development of the central nervous system or immune system, or both,” they said in a report published online by the British Medical Journal.

They added that the findings could partly explain the increased risk of MS in second-generation Asian and Caribbean migrants to the United Kingdom.

SOURCE: BMJ Online, 2004.