The Weather When You Were In Utero? Study Suggests it Influences Your Heart Attack Risk

By Ben Christopher

Fall and winter birthdays can be a real drag.

September celebrants will forever have their special days clouded by the beginning of a new school year. October through December birthdays are too closely clustered to all the good holidays, leaving the rest of the year a giftless wasteland. And while all the Geminis and Cancers get to have their parties in the park, the children of winter are forced to blow out their candles indoors and out of the gloom.

Now, thanks to an article soon to be published in Social Science and Medicine, end-of-the-year birth babies will have yet another reason to lament their inopportune DOBs: fall and winter birthdays might be bad for your health.

Okay, it might actually be a little more complicated than that.

The article, a joint effort of researchers from UC Irvine, UC Berkeley and Germany’s University of Mannheim, looks at an unlikely relationship between meteorology and cardiology. Specifically, the team compared daily temperature readings with the lifetime health records of 13,625 Swedes born between 1915 and 1929. By matching those babies to their corresponding death records through 2002, the researchers found a surprising, but significant, correlation: The more time a fetus spent in the womb during mild weather, the more likely that individual would die a cold-related death later in life.

In other words, mothers who spent the bulk of their pregnancies basking in the Swedish heat waves (Scandanavian “heat” being relative, with the top quintile pushing the mercury up to a raging 56 degrees Fahrenheit) were more likely to give birth to babies ill-equipped to deal with cold snaps later in life.

And that ought to be cause for concern, says Tim A. Bruckner, a professor of public health at UC Irvine and lead author on the study. Cold-related deaths, it turns out, are much more common than most would suspect.

“Cold kills more than heat does, unequivocally,” says Bruckner, who received his Ph.D in epidemiology at Berkeley in 2007. “In tropical, temperate and polar regions, cold comes out on top.”

No, there isn’t an unreported hypothermia epidemic. Instead, cold snaps can exacerbate underlying health conditions—particularly among the elderly and infirm. In cases other than death by sheer exposure, the relationship between cold and mortality is unexpectedly complex. But it is a near universal truth that when temperatures fall, circulatory and respiratory deaths rise accordingly—though, interestingly, that correlation is often much stronger in temperate climates such as Italy and Greece, compared to places where people actually know how to dress for the cold, such as Finland and Russia.

Because cold weather causes blood to thicken and arteries to constrict, the leading cause of cold-related death is ischemic heart disease, which includes heart attacks and angina. Strokes are not far behind. (One Dutch study from 1990 found that circulatory disease accounts for two-thirds of all cold-related deaths.)

According to Bruckner and his co-authors, who include Berkeley public health professors Ralph Catalano and Kirk Smith, that kind of heart disease is significantly more common in babies whose mothers enjoyed balmy pregnancies. Of the 173 Swedes who were felled by heart attacks in the midst of a cold snap, 76 of the dead had spent more than a quarter of their gestation periods in the warmest quarter of their birth year’s recorded daily temperatures.

In other words, warm weather babies made up 44 percent of all the cold weather heart attacks later in life. And that’s a bizarrely disproportionate share: only 30 percent of the entire 13,600-plus cohort belong in that warm-gestation category. 

In statistical terms, that would suggest that with every 6 percent increase in the amount of time a fetus spends in unseasonably warm weather, the risk of growing into an adult who will die of a cold-induced heart attack increased by 16 percent.

Oddly enough, strokes, which are another leading cause of cold-related death, were not found to be higher among the temperate gestation set. Bruckner speculates that this might simply be because the sample was too small, and that more research is required.

Indeed, this being the first study of its kind, Bruckner is hesitant to draw any sweeping conclusions.  But the trend is significant enough to suggest that there is some connection between extreme temperatures and the risk of heart disease.

Why on earth that might be so is another question entirely.

The de­vel­op­ing fetus can “an­ti­cip­ate” what kind of world it will be born in­to—and ad­just its de­vel­op­ment.“This is the equi­val­ent of the car-makers be­ing told the kind of con­di­tions the car will be driv­en in while they are build­ing it—it will be the same car, but its en­gine tun­ing, tires, and cli­mate con­trol sys­tem may change.”

“The theory I invoke it called ‘development plasticity,’ which is the idea that there might be certain innate or genetic characteristics that could be modified by exposure to certain environments,” says Bruckner. In other words, the hard wiring of our DNA may be a bit softer than we often think. Differences in ambient temperature—among any number of environmental stimuli such as exposure to certain chemicals or differing nutrient levels—can dramatically and permanently affect how a child develops. “This gets at the idea of ‘the long arm of childhood.’”

In their 2006 book, Mismatch: Why our World No Longer Fit Our Bodies, British developmental disease specialist Mark Hanson and New Zealand perinatal biologist Peter Gluckman explain how this “long arm” may in fact serve a unique purpose.  By responding to various environmental stimuli being passed to it through the mother, the developing fetus can “anticipate” what kind of world it will soon be born into—and will adjust its development accordingly.

“This is the equivalent of the car-makers being told the kind of conditions the car will be driven in while they are building it—it will be the same car, but its engine tuning, tires, and climate control system may change,” the authors note.

Bruckner uses the example of a pregnant mother who doesn’t get enough to eat. The fetus will develop to “expect” hard restraints on the amount of food it will find once it emerges into the world—and so the child might be born with a slower metabolism or a specific tolerance for sugar. (Anticipating limited gas supplies, the car-maker opts for a more fuel-efficient engine.) 

“You’d be relatively fine in a time of scarcity because you were prepared for it,” says Bruckner. “Then you confront a McDonald’s and that could be much worse than had you been born in times of abundance.”

This hypothetical gets at the “mismatch” referred to by Hanson and Gluckman. It is, says Bruckner, the “mismatch between the environment you’re planning to be born into and the one you actually experience.”

Bruckner and his colleagues hypothesize that those Swedish fetuses developing in warmer periods were primed to “expect” the good weather to continue. Unfortunately, fetuses are lousy at distinguishing long-term environmental trends from short-term anomalies. When the warm days turned out to be the latter, those babies grew into adults with biological characteristics poorly suited for Uppsala’s winters.

Complicating matters is the fact that fetal temperatures don’t actually fluctuate much, regardless of what’s going on outside. The human uterus has a remarkably good climate control system and so how the fetus is able to take a reading of the exterior climate is a mystery. Pregnancy is a bit of a “black box,” says Bruckner. Still, he and his co-authors have a few ideas.

First, there might be a slight hormonal shift that the mother experiences in response to temperature changes. Those hormones are passed on to the fetus, which uses them as cues to forecast its climactic future.

Another possibility is that the fetus reacts to changes in the mother’s blood flow, as it fluctuates with the cold.

Or, possibly, the developmental differences aren’t in response to temperature at all.

“This was in 1915, so perhaps it was a mother turning on the stove, which would result in increased coal fumes and so it’s actually an independent effect,” says Bruckner. “We can’t tease out the exact cause among so many possible explanations.”

Still, he says he’s encouraged by past research that has linked short-term fluctuations in temperature during gestation to later development in mammals. Studies have shown, for example, that after pregnant rats are exposed to the cold, their offspring are more likely to have thicker coats as adults.

Bruckner says a better understanding of the subject could be particularly important for immigrants who move from warm to cold climates—such as the many Turkish immigrants who live in Sweden. “For immigrant populations who aren’t used to bundling up—say, the many Somalis who live in Minnesota—this could be a big deal,” he says.

As for everyone else?

“Cold is a silent killer,” says Bruckner. No matter your fetal history, everyone should be more aware of this. He urges people, especially the elderly, to plan in advance, always check the weather, and be sure to bundle up. “Act like a Swede at all times.”

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