Ambient Temperature and Preterm Birth: A Retrospective Observational Study of 30 Million U.S. Singleton Births

摘要

Background: Emerging evidence suggests that prenatal exposure to temperature extremes may be associated with higher risk of preterm birth, but prior studies have been limited by small sample sizes and the results have been heterogeneous. We examined the associations between daily and trimester-mean ambient temperature during pregnancy and preterm birth in the contiguous United States. Methods: We linked 30,326,344 singleton births in 403 US counties between 1989 and 2002 to daily ambient temperature at the county level. Births delivered prior to 37 completed weeks of gestation were considered preterm. We used distributed lag non-linear time-series models to estimate the association of preterm birth with daily temperature in the 2 weeks prior to delivery, and logistic regression to assess the association between preterm birth and average temperatures in the 1st and 2nd trimester. Results: Approximately 2.7 million (9.0%) deliveries were preterm. Extreme heat (99th percentile versus median of county-specific temperatures) in the 2 days prior to delivery was associated with an incidence rate ratio (IRR) of preterm birth of 1.038 (95% CI: 1.027,1.050) while extreme cold (1st percentile versus median) was associated with an IRR of 0.984 (0.974, 0.994). Warm average temperatures (>90th percentile of county- and trimester-specific temperature distributions versus milder temperatures) during the 1st and 2nd trimester were associated with higher relative risk of preterm birth, with odds ratios of 1.025 (1.019,1.032) and 1.016 (1.009, 1.023), respectively, while cold average temperatures (<10th percentile) were associated with lower relative risks of preterm birth. Conclusions: Extreme heat - but not cold - during the 3rd trimester may be associated with triggering of preterm birth in the US. Warmer average temperatures in the 1st and 2nd trimesters may also be associated with higher relative risk of preterm birth. Heat may be a novel risk factor for preterm birth in the US.