Heat waves can be dangerous for anyone who doesn't drink enough water—but they pose particular risk for pregnant women. According to a new study, when temperatures exceed 89 degrees over a number of days, the risk for early delivery increases, Medical Express reports.
Researchers from the University of Montreal collected data from around 300,000 births that took place between 1981 and 2010, and compared them with temperature records from those delivery days. They controlled for other weather variables, as well as factors like the mother's age.
Heat, they found, did not increase the likelihood of a preterm delivery, but it did increase the risk of an early delivery. If temperatures remained in the 90s for three days in a row, a woman was 17 percent more likely to go into early labor, the authors found. If the weather remained hot from four to seven days, that risk increased 27 percent. As the study authors point out, these results are potentialy serious: infants born at 37 or 38 weeks are more likely to develop medical problems or even die, compared to their full-term counterparts.
The authors are not sure of why heat seems to trigger early delivery, Medical Express says. Another study found that uterine contractions seem to be triggered by heat, for example. Dehydration could have something to do with it, since a dehydrated mother would likely have reduced blood flow to her uterus.
Additionally, it's worth noting that the study took place in Montreal—a place where 97 degrees is the record summer high. Figuring out how these results would translate in more southerly locales, where summer temperatures regularly soar into the triple digits, will require additional studies.