Extreme heat may increase risk of early-term birth

When temperatures remain over 32° C for at least 4 days, the probability of an early-term delivery may increase by as much as 27%, according to new research in Epidemiology.

The findings are based on an analysis of the association between heat and risk of delivery of preterm and term pregnancies by researchers from the University of Montreal with a time-to-event design to minimize bias associated with seasonal variation on conception. Data on the 206,929 term and 12,390 preterm singleton live birth that occurred in Montreal from June through September, 1982 to 2010 were assessed.

Exposure variables considered were the maximum daily temperatures in the week before the delivery and the number of consecutive days where the temperature was ≥32° C. Hazards of delivery were estimated using Cox regression models for preterm (<37 gestational weeks), early-term (37-38 weeks), and full term (≥39 weeks), while adjusting for maternal characteristics. Sensitivity analyses were used to adjust for markers of air pollution.

Maximum daily temperatures reached at least 32° C in the preceding week for 19,829 births (9.0%). When compared to a maximum daily temperature of 20° C, the hazard of delivery within term was 4% higher when the temperature was ≥32°C. However, no association was found between preterm delivery and a daily temp ≥32° C, while associations were most strongly seen between early-term birth and higher temperatures. Early-term deliveries were associated with a 27% greater risk when temperatures ≥32° C were seen for 4 to 7 days.

The investigators concluded that extreme heat episodes and high ambient temperatures may trigger early-term births. 

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