Why People Born in Winter Might Live Longer

An analysis of Swedes shows that those who gestated and were born during warmer times were more likely to die of cold-induced heart attacks.
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David Salafia/flickr

Extreme cold kills more people than extreme heat, and it does so in a variety of ways.

You could freeze to death. You could be spending more time inside, picking up all sorts of nasty respiratory infections. More often, though, frigid temperatures get you in an even sneakier way: Cold weather causes arteries to constrict and blood to become thicker, increasing chances of having a heart attack or stroke. The winter months usually see a peak in various types of heart diseases, including heart attacks.

Weight, fitness, and lifestyle factors all contribute to the likelihood of having a heart attack during a cold snap, of course. But now, it looks like there's another cause—one far beyond your control. There’s evidence that your risk of dying of heart disease in the cold could depend on the temperature at which you experienced life as a fetus.

Proportion of warm days in gestation vs. likelihood of heart attack
during a cold spell. (Social Science in Medicine)

A forthcoming study in Social Science and Medicine shows that people who were in utero during the warmer months were more likely to die from this type of wintertime heart disease during cold periods.

For the study, researchers from University of California at Irvine, the University of California at Berkeley, and the University of Mannheim, Germany examined 13,500 Swedes born in Uppsala, a town just north of Stockholm, between 1915 and 2002. They then matched up their birthdays with the outside temperatures during the times they were gestating.

They found that the likelihood that the person would go on to die a cold-related heart-attack death was 16 percent higher for every 6 percent increase in the number of warm days they spent in utero. (Uppsala is not very warm, granted. The study viewed “warm” days as those that were above 56 degrees F.)

What's more, for the Swedes born during a warm time, the risk of a heart-disease death was higher in a cold spell than in a warm one. In other words, if you were born in August and you hear the word "flurries" on the news, clutch the nearest space heater and don't let go.

What’s the reason for this odd association? One theory is the developmental plasticity hypothesis, or the idea that these warm-weather gestaters have mismatch between the signals they received in utero and their experiences in real life. Not unlike a spoiled millennial who is about to enter the real world after breezing through Cornell, this zygote is in for a rude awakening.

There is not much that Swedes—or anyone—can do about the month in which they are conceived, but it’s an interesting takeaway for people who immigrate from more temperate climates to colder ones. It seems like the better idea would be to go in the opposite direction: There’s a theory that the ongoing migration from the Northeastern U.S. to Southern states saves 5,400 lives a year.

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Olga Khazan is a staff writer at The Atlantic, where she covers health.

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