There are many studies that show that the risks for getting multiple sclerosis (MS) vary according to the month a person is born. However, the differences between the months of birth are slight.
For example, a 2005 study of people with MS living in northern latitudes found that more people (9.1%) had a birthday in May and significantly less (8.5%) were born in November. The opposite pattern is seen in the southern hemisphere. Thus, worldwide there is a slight increase in MS risk in those born in the spring and a decrease in those born in the winter.
The cause of this has not been determined. Some ideas include differences in:
Vitamin intake during pregnancy (more folate in fresh vegetables in the spring, more vitamin D from sunlight in the summer)
- Birth weight - Heavier babies born after summer and fall pregnancies
- Exposure to viruses - More people experience viruses in spring and fall. This may affect the not only the viruses a baby is exposed to during pregnancy, but also after birth.
A recent article in JAMA Neurology describes a new study of how birth month may affect risk. Researchers measured of immune function in umbilical cord blood obtained from 50 babies born in November and 50 in May. All were from London. They found a higher level of immune cells (T lymphocytes) in the cord blood from babies born in November compared to those born in May. They also found higher levels of vitamin D in babies born in November compared to those born in May.
These findings are interesting because they confirm that there are changes in nutrition, vitamin exposure and immune changes related to birth month. It is not clear how these changes might alter the risk of MS because all of these babies were healthy. Babies in this study were born in 2009-2010 and are just 3-4 years old. We do not know how many, if any, will develop MS and whether any of these factors truly change the risk of getting the disease. Nevertheless, these are important clues that are being pursued in the search for the cause of MS.