A new study shows a link between warmer temperatures and higher rates of gestational diabetes.

Researchers have identified a new possible risk factor for gestational diabetes: Being exposed to hot outdoor temperatures in the month before giving birth. According to a new study published in the Canadian Medical Association Journal (CMAJ), in one geographic region in Canada, the rate of gestational diabetes varied more than three percentage points between the coldest times of year and the hottest.

Gestational diabetes, a condition that develops during pregnancy, is usually a temporary condition. But women who develop it are at higher risk for type 2 diabetes and other health conditions later on. (One recent study linked gestational diabetes to postpartum depression, for example.) Gestational diabetes can also raise babies’ risks of excessive birth weight, preterm birth, and type 2 diabetes later in life.

Previous studies have suggested that exposure to cold temperatures can improve insulin sensitivity and activate the body’s brown fat tissue, which—unlike other types of fat—burns calories and seem to protect against metabolic conditions like obesity and diabetes. For this reason, it’s been suggested that cold temperatures might protect against these conditions.

To study this potential connection in pregnant women, researchers looked at medical records of nearly 400,000 women living in the same urban area in Canada, in a region with wide temperature fluctuations throughout the year. Over a 12-year period, those women gave birth to more than 555,000 children.

When the researchers looked at average temperatures in the 30 days before these women gave birth, they found that the prevalence of gestational diabetes was 4.6% among those exposed to very cold temperatures (14 degrees Fahrenheit or lower), compared to 7.7% among those exposed to very hot temperatures (75 degrees Fahrenheit or higher).

That translates to a 6% to 9% relative increase in the risk of gestational diabetes for every increase of about 18 degrees Fahrenheit (or every 10 degrees Celsius), the researchers calculated. The relationship held true even when the researchers compared two consecutive pregnancies in the same woman, which allowed them to cancel out factors such as income, ethnicity, and lifestyle habits that could also affect risk.

This may seem counterintuitive, says lead author Gillian Booth, MD, a researcher at St. Michael’s Hospital and the Institute for Clinical Evaluative Sciences in Toronto, since people might think that pregnant women would spend more time outdoors and be more active during warmer months. But the findings fit a pattern of recent research on the potential protective effects of colder temperatures, she says.

And although the researchers only looked at a single geographic region in Canada, they say the results could likely apply to other parts of North American and the world. “Based on the study’s findings we would expect pregnancies in warmer climates to be at higher risk of gestational diabetes, although we weren’t able to look at that directly,” Dr. Booth told Health. “I think that’s a great topic for future research.”

Likewise, the researchers suspect that summer pregnancies could be riskier than winter pregnancies—and they’re not the first to make this claim, either. “Although we didn’t look at seasons per se, rates were highest when that 30-day window happened in the summer when the temperature was hottest,” Dr. Booth says.

The study also suggests that gestational diabetes—and adult-onset diabetes, which shares the same risk factors—could become increasingly prevalent worldwide as global temperatures continue to increase.

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Because the study was not a randomized trial, it was unable to show a cause-and-effect relationship. The study also did not include information about body mass index, weight gain, physical activity, or diet of the women involved. But it does “offer fairly compelling evidence that air temperature may be a modifiable risk factor for gestational diabetes,” says Dr. Booth.

Strategies like using air conditioning and avoiding excess layers in the summer might influence a pregnant woman’s risk of gestational diabetes, says Dr. Booth. These may be especially important for women who have strong non-modifiable risk factors for gestational diabetes, she adds, including non-European ethnicity, family history of type 2 diabetes, or a prior pregnancy with gestational diabetes.

Temperature appears to have a smaller effect on gestational diabetes than these other risk factors, but “it is one factor that might be sufficient to push someone’s risk over the edge,” says Dr. Booth. And while the number of women impacted globally by temperature fluctuations may be substantial, she says, the increased risk for individual women is likely small.

Staying cool during hot weather is probably a smart idea, says Dr. Booth, but women can also protect themselves from gestational diabetes by following strategies that have been proven to lower their risk—like achieving a healthier body weight before they conceive, and avoiding excess weight gain during pregnancy by consuming a healthy diet and being physically active.