Does the month you were born affect your risk of developing MS?


19 July 2016

The study in brief

The study undertook a large and very detailed survey to determine whether there is a pattern to birth month for people with multiple sclerosis.

This research, which involved more than 21,000 people with MS found that people diagnosed with MS are more likely than average to have been born in April, and less likely than average to have been born in November. For people born between 1938 and 1980, there were 6.7% more MS births than expected in April, and 9% fewer MS births than expected in November. A similar pattern was found for people born between 1965 and 1980.

The results of the study suggest that environmental factors very early in life contribute to the risk of MS.  One of these factors could be low sunlight exposure during winter pregnancies, leading to low maternal vitamin D levels which in some way increase the risk of developing MS later in life. Other factors such as the mother's exposure to seasonal infections or seasonal differences in diet could also play a part.

The study in detail

Background

If month of birth has no effect on subsequent diagnosis, you would expect to see a similar number of people with MS born each month throughout the year. Previous studies have investigated this but have been criticised for the numbers of people included in the study or for failing to take account of background trends in the general population. For example, there are known to be seasonal variations in birth rates in the general population and patterns have changed over the years. Regional and latitude variations in birth month add extra complexity which needs to be factored into statistical calculations.

This study aimed to get a clearer picture of the distribution of birth months for people diagnosed with MS by carrying out a meticulous analysis which fully reflected background variations in the general population.

How this study was carried out

Data from more than 21,000 people with MS was provided by eight MS specialist centres in the UK and from a study of the prevalence and incidence of MS in the Borders and Lothian regions of Scotland. Data for the general population was obtained from the UK Office of National Statistics. The researchers made detailed adjustments to allow for background variations which could influence the results, taking account of patterns of births in the general population, reflecting year of birth, latitude and region.

What was found

Analysis of the general population indicated that there are seasonal differences in the monthly distribution of births which have varied over time and for different regions of the United Kingdom.

After accounting for background variations in the general population, there was a significant season of birth effect, with more people diagnosed with MS having birthdays in the spring, peaking in April, and fewer people with MS having birthdays in the autumn, peaking in November. For people born between 1938 and 1980, there were 6.7% more MS births than would be expected in April, and 9% fewer MS births than would be expected in November. A similar pattern was found for people born between 1965 and 1980.

The data suggest that there is an increased risk of developing MS if you are born in the spring and a reduced risk of developing MS if you are born in the autumn.

What does it mean?

The causes of MS still remain unclear. It is generally agreed that some people have a genetic make-up that predisposes them to MS and that one or more environmental factors act as a trigger which sets off a chain of events leading to them developing MS.

Establishing that there is a genuine difference in the risk of developing MS for people born at different times of the year suggests that some environmental risk factors could act before birth and/or in early life. One explanation that has been proposed is that low sunlight exposure during winter pregnancies leads to low maternal vitamin D levels which in some way increase the risk of developing MS later in life. Other factors such as the mother's exposure to seasonal infections, seasonal differences in diet or in activity levels could also play a part.

A better understanding of the mechanism by which birth month influences MS risk could help to identify ways to reduce the risk and could have consequences for the timing of strategies to prevent MS.

Comment

Birth month is only one of a number of environmental factors that could influence the risk of developing MS. Other factors which may be important include smoking and exposure to infections, especially Epstein-Barr virus which causes glandular fever.

Rodríguez Cruz PM, Matthews L, Boggild M, et al.
Time- and region-specific season of birth effects in multiple sclerosis in the United Kingdom.
JAMA Neurol. 2016 Jun 27. doi: 10.1001/jamaneurol.2016.1463. [Epub ahead of print]

More about causes of MS

You can read more about the causes of MS, the risk of developing MS and the incidence and prevalence of MS in the A-Z pages of the MS Trust website.

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