Birth before 37 weeks of gestation is a leading cause of infant illness and death. The reasons for it are multiple and complex, but some previous studies have found evidence that one contributor could be extreme hot weather.

This new study, in Environment International, tracked nearly 2 million singleton pregnancies in California during the summer months from 2005 to 2013, comparing the duration of gestation with temperature records by ZIP code. Overall, 6.72 per cent of births were premature.

The researchers used four relative temperature thresholds to define heat waves, ranging from the 75th to the 95th percentile of all the maximum temperatures in each ZIP code (a “heat wave” in San Francisco, for example, where temperatures rarely exceed 90 degrees, would have a much lower temperature than one in the much hotter region of Palm Springs, where temperatures can soar well above 110). Within each percentile category, they recorded whether the heat wave lasted two, three or four days or more.

With each increase in temperature or duration of the heat wave, the percentage of preterm births increased steadily. At a mean temperature of 88 degrees for two days, for example, 6.63 per cent of births were preterm. At four days of 98-degree temperature, the rate was 7.46 per cent.

The reasons are not clear. “Extreme heat might make mothers more susceptible to changes in hormones,” says the lead author, Sindana D. Ilango, a graduate student at the University of California, San Diego. “Dehydration could also play a role. These factors may trigger contractions and earlier birth.”

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