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Exposure to Heat Wave During Pregnancy and Adverse Birth Outcomes: An Exploration of Susceptible Windows.

BACKGROUND
Several studies have investigated the acute effects of high ambient temperature or extreme weather on preterm birth and stillbirth. However, little was known about whether there are any particular stages during which high ambient temperature or heat wave exposure is most harmful to fetal growth. The aim of this study was to examine the effects of heat wave exposure in different gestational months on the risk of preterm birth and stillbirth.


METHODS
Based on all singleton birth records between 2000 and 2010 and the corresponding environmental data in Brisbane, Australia, we explored relationships between heat wave exposure in different months of pregnancy and preterm birth or stillbirth. Six heat wave definitions were used in this study. After controlling for confounding factors, hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using extended Cox proportional hazards regression models with time-dependent covariates.


RESULTS
For most heat wave definitions, the adjusted HRs of preterm birth varied by different gestational months and ranged from 1.08 (HR = 1.08; 95% CI = 1.00, 1.18) to 1.53 (HR = 1.53; 95% CI = 1.41, 1.68). Heat wave exposure in early pregnancy was more likely to increase the risk of stillbirth compared with heat wave exposure in late pregnancy.


CONCLUSIONS
Overall, we found relationships between preterm birth and heat wave exposure in all months of pregnancy based on most heat wave definitions. We did not find a specifically high period of susceptibility during pregnancy for preterm birth associated with heat wave exposure. However, earlier gestational months might be key exposure windows for heat-wave-affected stillbirth.