The month or season during which an individual is born may affect the risk of different diseases such as diabetes, or asthma. This effect is the result of environmental factors that are not constant over the year for the mother and the unborn baby.

For example, babies born in the UK in April have been exposed to less sunshine (less vitamin D) through their mothers, than those born in November. Therefore, being born during spring (March, April, and May) may increase the risk of developing MS. However, other studies have challenged these findings underlining more complex factors that may affect the risk of MS, which may not be easily explained by the season of birth.

In this large study which included more than 21,000 people with MS, authors from Oxford University, UK, asked whether the month of birth had any effect on the risk of developing MS across the UK. They found that there is a significant effect of month of birth on the risk of MS.

Babies who were born during April (peak) had the highest risk of being affected by MS later in life, and those born during November (trough) had the lowest risk (with a 17-24% risk difference between April when compared to November).

This can be explained by, for example, mothers being exposed to less sunshine (and vitamin D) during winter months (December, January, and February), different physical activities or diets over the pregnancy period, etc. This effect, however, is not specific to MS and other disorders have similar associations with the month during which an individual is born.

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