Researchers have suggested that when temperatures reach 32 degree Celsius or higher over a period of four to seven days, the risk of early-term delivery is 27 per cent higher than on typical summer days.
The study led by Nathalie Auger of the University of Montreal's Department of Social and Preventive Medicine involved data from 300,000 births that took place in Montreal between 1981 to 2010 with summer temperatures recorded by Environment Canada during this period.
The research team sought to identify, from June through September, the probability of preterm (less than 37 gestational weeks), early-term (37-38 weeks), and full-term (39 weeks or more) deliveries during a heat wave. Specifically targeting summer births, Auger found that nearly 20,000 deliveries occurred in the week following a day when the mercury reached 32 degree Celsius or higher.
After adjusting for certain variables, including age of mother, birth order, and humidity during heat waves, Auger found that extreme heat did not seem to increase the number of preterm births.
"We observed a only a negligible increase in the rate of preterm births between days when the temperature was below 20 degree Celsius and those when it was above 28 degree Celsius, from 5.4 per cent to 5.8 per cent," Auger explained. However, in women who reached 37 or 38 weeks of pregnancy, the risk of early-term delivery increased by 17 per cent following a three-day episode of 32 degree Celsius or more, compared to days without a heat wave. When the extreme heat episode lasted from 4 to 7 days, the risk reached 27 per cent.
Auger believes that the increased risk of early-term delivery due to high temperatures may result in increased morbidity in newborns.
