A new study has revealed that the risk of developing multiple sclerosis changes with the season.
Lead researchers Emmanuelle Waubant and Ellen Mowry revealed that this seasonal effect is mediated by gene HLA-DRB1.
In many European populations, the HLA-DRB1*15 allele of this gene is associated with an increased risk of MS, and the large-scale study of MS patients from Canada, Sweden and Norway now shows that this allele is more common among patients born in the spring.
The researchers said the study was "unique in its attempt to understand how genes and environment interact in MS".
They said Vitamin D appears to influence the expression of the HLA-DRB1*15 allele.
Since vitamin D production fluctuates with the seasons, a vitamin D deficit in pregnant mothers could be related to the increased risk of MS among spring births.
However, further studies are required.
Waubant and Mowry said that understanding the environmental risks and their interaction with relevant genotypes might pave way for new treatments.
Study author Dr Sreeram Ramagopalan said that taking vitamin D supplements during pregnancy may reduce the risk of a child developing MS in later life.
Government guidelines also recommend that children under five take daily vitamin D supplements.
The findings appear in journal Neurology.