Doctors claim the month in which you are born has a direct impact on your risk of developing multiple sclerosis later in life. That's the focus of a new study by researchers in the United States, Canada and Great Britain, published last month in the British Medical Journal.1
Environment Plays a Role
The researchers found that of those living in the northern hemisphere, being born in May is linked to an increased risk, while having a birthday in November carries the lowest risk.
"Classic studies of twins, adoptees, half siblings, and families have led to a widely accepted notion that multiple sclerosis (MS) is a complex trait in which susceptibility is determined by the interplay of genes and environmental factors," wrote Jock Murray, MD, a professor of Medicine at Dalhousie University in Halifax, Nova Scotia and his colleagues.
"In Canada, the risk for a dizygotic twin of someone with MS is almost twice that for a full non-twin sibling," the study team wrote. "This may implicate environmental factor(s), such as shared timing of gestation or birth, or both. Furthermore, the study of half siblings has shown a maternal effect that could be environmental in nature."
Evaluating Timing of Birth to MS Risk
To focus on how the environment interacts with certain genes in people more susceptible to developing MS, Murray's team focused on nearly 18,000 Canadian patients and about 11,000 British patients with multiple sclerosis for their study. Information on the month of birth, along with detailed data on demographics and medical and family history were collected by Murray's group and studied extensively. Information on the MS patients was compared to that of the general population and unaffected brothers and sisters of those with MS.
The investigators learned that in Canada, significantly fewer people with MS were born in November compared with those in the general population and in the group of unaffected siblings, defined as the control group. The number of those born in December was also significantly lower. By contrast, significantly more people in the study group were born in May.
"Among Canadian patients with MS, 8.5 percent fewer than expected were born in November compared with other months, and slightly but not significantly more were born in May," Murray and his group wrote. "This finding was replicated in British patients with MS, where the numbers born in both months were significantly different from controls."
Overall, patients in both countries faced a 13% increase in the risk of MS for those born in May compared with November. The effect was most evident in Scotland, where the prevalence of MS is highest.
The Causes Have Yet to be Uncovered
While the exact reasons for the increased risk for those born in May aren't exactly known, the researchers speculate that somehow, the risk factors vary seasonally, and may interact in some way with the development of the central nervous system before birth. Additionally, previously published research has suggested that exposure to the sun or seasonal variations in a mother's vitamin D levels during pregnancy may have an impact on brain development.
It's known that vitamin D, which is gathered through the skin in sunlight, reduces the severity of symptoms and progression of an MS-like disease used in animal research. The study authors theorize that mothers may have less exposure to vitamin D in northern countries where the sun is less intense.
"These findings support suggestions from studies in twins and half siblings that the gestational or neonatal environment, or both, influence the risk of MS later in life," Murray and his fellow investigators wrote.
1. Willer CJ, Dyment DA, Sadovnick D, Rothwell PM, Murray TJ, Ebers GC. Timing of birth and risk of multiple sclerosis: population based study. BMJ Epub 2004 Dec 7.
John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include coverage of health news for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.
