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Month of Birth and MS

Last updated: January 2023

Twelve new months. In which were you born?

As we know, MS is still one of those diseases where both cause and cure continue to be a mystery. In order to learn more about MS, epidemiologists can often find clues that help guide scientists in other fields. At its most basic level, epidemiology is a research field where large sets of information about many people are collected and analyzed with the goal of uncovering some type of common thread within a group of people who have a particular disease (for example, cigarette smokers and lung cancer).

The role of epidemiology

The field of epidemiology stepped to the front lines during the COVID-19 pandemic. By gathering information about the location, age, and symptoms of large numbers of people, epidemiologists showed us where and how to expect the virus to spread, which people would be most at risk (such as people suffering from immune system problems), the impact of wearing masks, and the fact that the coronavirus could be spread by an infected person who showed no symptoms.1

What have we learned about people with MS?

From epidemiological studies of MS patients, we have learned things such as:

  • Almost all people who develop MS have first had the Epstein-Barr virus (commonly called mononucleosis or “mono” in college dorms).2
  • Most people with MS show vitamin D deficiency. (You can check for this on your basic blood panel.)3
  • Worldwide, twice as many women have MS as men.4
  • The percentage of people with MS increases with distance from the equator. (For example, North America has a higher percentage of people with MS versus South America. Even within Ireland, there is a higher percentage of people living with MS in the northern versus the southern regions.)5,6
  • People with MS are more likely to be born in the springtime and less likely to be born in late fall/early winter.7

What is the MS month of birth effect?

Wait. What was that last one there?

Yes, over the past 20 years, MS research groups from Canada to Norway to the United Kingdom looked at their country’s birth records and found that the people who have developed MS have a significantly different seasonal birth pattern than each country’s general population. People with MS were most often born in March, April, and May and least often born in October and November.8

This epidemiological factor is known as “seasonal factors” or (my preferred term) “MoBE”: the Month of Birth Effect.

But...why?

Location, location, location

Given that patterns have been found in the location of birth (closer or farther from the equator) as well as in low vitamin D, researchers often try to explain the month of birth effect in MS by connecting it to ultraviolet light exposure and related vitamin D levels during pregnancy. In other words, if a woman who lives in a more northern region is pregnant during the winter months, then her body will be exposed to fewer hours of direct sunlight.9

There is still a lot to learn

Okay, before we go blaming everything on Mom or our distance from year-round sunny weather, we must remember that researchers are truly still in the dark as to how or whether MS might be caused by some combination of Epstein-Barr infection, geographic location, environment, and/or a possible inherited risk of developing MS.10

Other seasonal considerations

Returning to the MoBE data for now, I think that we can all agree that the number of potential hours of exposure to sunlight per day is not the only thing that changes with the seasons. Personally, as a farmers’ daughter by birth and a scientist by training — which actually lead to some very overlapping areas of know-how — my understanding of seasonal differences only begins at sun exposure.

Food sources and choices

The type of food that can be locally produced changes by seasons and the length of seasons, as does the type and timing of pesticide application. Some countries consistently import fresh fruits and vegetables from other locations during winter months, introducing a whole new ensemble of pesticides, growth strategies for lengthy shipping times, etc. Having known friends while they were pregnant (and having a quick check of the Johns Hopkins recommendations), I feel safe in suggesting that many women eat quite differently when they are pregnant.11

What if the food available on the grocery store shelves is very different in the fall (the first trimester for a baby to be born in the spring) than it is in the springtime (the first trimester for a baby to be born in early winter)? And then there are latitude-impacted seasonal changes to water supply, for both private wells and municipal sources.

What do you think of the MoBE?

So, as we begin another 12 months, what do you think about the month of birth effect and MS?

What month were you born?

I was born in late March, in Pennsylvania.

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