High Ambient Temperature and the Risk of Preterm Delivery

Abstract

A number of studies have linked preterm delivery to environmental factors such as air pollution and traffic exposure. The few studies examining the associations between preterm delivery and temperature or other meteorologic variables have been inconclusive. This population-based epidemiologic study assessed the possible association between preterm delivery, and heat and humidity (measured by apparent temperature during the warm season in California from 1999 to 2006). The investigators performed a case-crossover analysis of almost 60,000 births occurring in 16 counties between May and September. Cases of preterm birth were identified using the state registry of births and were combined with meteorologic and air pollution monitoring data (based on the zip code closest to the mother's residence). There was a significant positive association between high ambient temperature and preterm delivery among all women in vulnerable subgroups (stratified by maternal age, race/ethnicity, level of educational attainment, or infant sex). An 8.6% increase in preterm delivery (95% confidence interval, 6.0–11.3) was associated with a weekly average increase (lag06) in apparent temperature of 10°F (5.6°C). Greater elevated risk was found for younger maternal age groups, African Americans, and Asians. The change in temperature associated with preterm delivery was independent of air pollutants. These findings suggest the need for future large scale studies to fully evaluate the positive association between apparent temperature and preterm delivery shown in the present study.