A heatwave can cause more than just discomfort. New research now shows that it can also cause preterm birth. Published in the journal Environment International showed that exposure to extreme heat in the final weeks of gestation can increase the risk of preterm birth. Since preterm birth is a leading cause of infant mortality and morbidity, understanding triggers like heat can help us identify triggers and establish warning systems. 
The findings are based on a population-based cohort study. The research team studied 1,967,300 women in California from May 2005 to September 2013. They created 12 categories of extreme heat as per the relative temperatures and durations. The temperatures were as per the 75th, 90th, 95th, and 98th percentiles and the durations were 2, 3, and 4 consecutive days. Daily temperatures were taken as per the zip code of the participant residences. At less than 37 weeks, the risk of preterm birth was estimated in relation to exposure to extreme heat.
The team found that roughly 7 percent of the cohort gave preterm birth. More importantly, the risk of preterm birth was consistently higher for women who were exposed to extreme heat in the last weeks of gestation. They also found that the duration was even more important than the temperature. This could be used to design warning systems for people living in extreme heat zones.