Gestational diabetes is a birth complication affecting millions of women worldwide. New research examines the link between air temperature and the risk of developing this condition.

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New research suggests that hotter outdoor air temperatures may significantly increase a mother's risk of GD during her pregnancy.

Gestational diabetes (GD) is a temporary form of diabetes that affects some women during pregnancy. These future mothers often have no history of diabetes prior to getting pregnant, but their blood sugar increases by the time they are halfway through their pregnancy.

In 2014, the Centers for Disease Control and Prevention (CDC) reported that in the United States, as many as 9.2 percent of pregnant women may be affected by GD. Worldwide, around 1 in 7 births are affected by this complication.

GD occurs when the hormones in the placenta trigger insulin resistance, meaning that healthy blood sugar levels cannot be maintained.

Sensitivity to insulin is known to be improved by exposure to cold temperatures, during which the body produces more heat to keep itself warm. However, could it be that outside air temperatures have a larger impact on the risk of developing GD?

Researchers from St. Michael's Hospital, the Institute for Clinical Evaluative Sciences (ICES), Mount Sinai Hospital, and the University of Toronto - all in Ontario, in Canada - set out to explore the connection between outdoor air temperature and the risk of GD.

The findings were published in CMAJ.

Each 10°C temperature increase raises risk of GD by 6 to 9 percent

The study examined 555,911 births from 396,828 women over a period of 12 years, from 2002 to 2014.

The women were 31 years old on average, and lived in the Greater Toronto Area. Approximately half of the mothers were not born in Canada.

Extremely cold outdoor temperatures were defined as an average of 10°C or less, and hot temperatures as 24°C on average.

The women were exposed to these different temperatures for 30 days before being screened for GD.

GD occurred in 4.6 percent of the women that had been exposed to extremely cold temperatures, but this number jumped to 7.7 percent for the women who had been exposed to hot temperatures.

Furthermore, each 10°C increase in temperature correlated with a 1.06 times higher risk of GD. A similar trend was found when the temperature increased between two pregnancies in the same mother.

"By further limiting our analysis to pregnancies within the same woman, we controlled for a whole number of factors," says co-lead author Dr. Joel Ray, a researcher at St. Michael's and ICES. "Doing so allowed us to eliminate factors like ethnicity, income, activity, and eating habits that would differ between two different women."

Lead author Dr. Gillian Booth, a researcher at St. Michael's and ICES, reports on the findings:

"We observed a direct relation between outdoor temperature and the risk of gestational diabetes among nearly 400,000 women residing in a single urban area in Canada.

After adjusting for influential risk factors, each 10°C increase in [...] temperature was associated with a 6-9 percent relative increase in the risk of gestational diabetes mellitus."

Cold exposure can improve insulin sensitivity

Although the study was observational and cannot provide causality, Dr. Booth offers a potential explanation for the seemingly counterintuitive results:

"Many would think that in warmer temperatures, women are outside and more active, which would help limit the weight gain in pregnancy that predisposes a woman to gestational diabetes. However, it fits a pattern we expected from new studies showing that cold exposure can improve your sensitivity to insulin, by turning on a protective type of fat called brown adipose tissue."

"Although we studied a single geographical region, our findings are likely to be generalizable to other regions in North America and worldwide," say the authors.

They also warn that, if their findings are correct, this could potentially mean that the worldwide number of GD cases might continue to increase as a consequence of global warming.

"Although changes in temperature of this size may lead to a small relative increase in the risk of gestational diabetes mellitus, the absolute number of women affected in Canada and elsewhere may be substantial," the authors conclude.

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