Exposure to extreme temperatures may increase the risk of preterm birth: findings from a contemporary US cohort.

Abstract

Background: The frequency and severity of extreme temperatures have been increasing and are expected to continue to escalate in the coming decades. Relationships between extreme temperatures and early deliveries are not well understood.

Methods: Data were obtained from 201 759 pregnancies with gestational age (GA) of >27 weeks in the Consortium on Safe Labor retrospective cohort study from 19 hospitals across the USA for 2002-8. Location-specific bulk temperatures were obtained from the Parameter-Elevation Regressions on Independent Slopes Model (4-km resolution), from which participants were linked to minimum, maximum, and mean daily temperatures across each day of pregnancy. Days were categorized for each site as cold (both minimum and maximum temperatures ≤ 20th percentile), hot (both minimum and maximum temperatures ≥ 80th percentile), or neither for each pregnancy. Cold and hot days were then analysed individually or as the proportion within specific pregnancy windows by using Cox regression and fetuses-at-risk logistic regression models.

Results: Detrimental relations in fetuses-at-risk models were consistently observed between exposure to chronic cold and chronic heat and early delivery: odds ratio (ORcold)= 1.66, 1.40, and 1.16; ORheat = 1.20, 1.13, and 1.32 for early-preterm (GA < 34 weeks), preterm (34 ≤ GA < 37 weeks), and early-term (37 ≤ GA < 39), respectively. Associations for exposures in Trimesters 1 and 2 were mixed but clearly more detrimental in Trimester 1. The likelihood of early delivery increased with daily exposure to extreme temperatures after 27 weeks.

Conclusion: Future work should seek to elucidate the underlying mechanisms and to validate these findings by using more recent diverse global data.