Association between maternal exposure to extreme heat and the risk of preterm birth in China: a sibling-matched study

Abstract

BACKGROUND AND AIM: Extreme heat exposure during pregnancy may increase the risk of adverse birth outcomes. This study examines the association between extreme heat and preterm birth (PTB) using the novel sibling-matched design which has the potential to eliminate the effects of unknown and unmeasurable confounders. METHODS: Using a large population-based birth cohort across 16 counties in China, we collected 10,826 sibling pairs (21,652 singleton live births) from March 2013 to December 2018. Daily mean temperature was collected in each county and extreme heat was defined as temperatures above the 90th percentile of trimester- and location-specific mean temperature. Conditional logistic regression models were used to estimate the effects of extreme heat exposure during each trimester and for the entire pregnancy on PTB. Stratified analyses were performed by regional characteristics, birth order, prenatal care status, inter-pregnancy interval, and delivery mode. RESULTS:Of the 21,652 births (10,826 pairs) included in the study, 804 (3.71%) cases were PTB, and 29 sibling pairs were both PTB. Contrasting with normal temperature exposure, heat exposure during the 3rd trimester and entire pregnancy was associated with PTB risk, with Odds Ratios (ORs) of 2.34 (95% CI: 1.65, 3.33) and 3.21 (95% CI: 2.23, 4.61), respectively. Women who lived in areas with lower levels of health services had a higher risk of PTB (OR: 4.28, 95% CI: 1.72-12.72). Heat-related PTBs were also more likely to be observed for first-borns and for second-born neonates with an inter-pregnancy interval of less than 18 months; ORs (95% CIs) were 4.59 (3.39, 6.22) and 3.43 (2.16, 5.40), respectively. Insufficient prenatal care also increased the risk of PTB associated with heat exposure. CONCLUSIONS:Using a sibling-matched design, our research strengthens the evidence that extreme heat exposure during pregnancy increases the risk of PTB. Our findings for susceptible subgroups could also help to improve guidance for prenatal care in China. KEYWORDS: Climate, Temperature extremes, Birth outcomes, Environmental epidemiology