Pregnancies, heat waves don’t mix

Risk increased by up to 27 per cent

A Université de Montréal study examining summer heat waves and pregnancy discovers that a spike in temperature lasting several days increases the risk of early term delivery by as much as 27 per cent. Researchers are not sure what triggers labour, but it could be related to heat stress or dehydration.

It might seem premature to speak about the effects of heat waves at a time when Montreal is barely emerging from the chill of winter.

However, a Montreal study looking at extreme heat and pregnancy shows that a spike in temperature over several days increases the risk of early-term delivery.

The study, led by researcher Nathalie Auger of the Université de Montréal’s department of social and preventive medicine, notes that the impact of high heat and dehydration on the very young and the elderly is well known, but few studies have evaluated another potentially vulnerable population: pregnant women.

Auger’s team collected data from nearly 300,000 live births in Montreal during the summers between 1981 and 2010.

The study found no association between high temperatures and preterm babies. But a persistent heat spell could trigger early labour. The likelihood of early deliveries rose as temperatures soared during the final weeks of pregnancy, researchers found.

Nearly 20,000 babies were born after a series of hot days when the mercury hit 32 C.

In women at 37 or 38 weeks of pregnancy, the risk of early-term delivery increased by 17 per cent after a threeday episode of 32 C or higher, compared with typical summer days without a heat wave. When the heat spell lasted from four to seven days, the risk was 27 per cent higher.

The authors note that the health impact of early deliveries is unclear, but that babies born at 37 and 38 weeks of gestation are more vulnerable to developing breathing difficulties and other medical problems, and even dying, compared with babies born at full term (40 weeks).

Auger’s team controlled for factors other than heat, such as the mothers’ ages, plus weather variables and air pollution.

Auger’s team is not sure why heat extremes trigger early deliveries. They note that other small-scale studies found that heat stress increases uterine contractions. Dehydration due to heat could also reduce blood flow to the uterus and that may trigger labour.

Auger is affiliated with the university’s superhospital research centre. Published online in Epidemiology, the study was funded by the Fonds de recherche du Québec–Santé.