Maternal exposure to ambient temperature and the risks of preterm birth and stillbirth in Brisbane, Australia.

Almost 10% of all births are preterm, and 2.2% are stillbirths. Recent research has suggested that environmental factors may be a contributory cause of these adverse birth outcomes. The authors examined the relation between ambient temperature and preterm birth and stillbirth in Brisbane, Australia, between 2005 and 2009 (n = 101,870). They used a Cox proportional hazards model with livebirth and stillbirth as competing risks. They also examined whether there were periods in pregnancy where exposure to high temperatures had a greater effect. Higher ambient temperatures in the last 4 weeks of the pregnancy increased the risk of stillbirth. The hazard ratio for stillbirth was 0.3 at 12°C relative to the reference temperature of 21°C. The temperature effect was greatest at less than 36 weeks of gestation. There was an association between higher temperature and shorter gestation, as the hazard ratio for livebirth was 0.96 at 15°C and 1.02 at 25°C. This effect was greatest at later gestational ages. These results provide strong evidence of an association between increased temperature and increased risk of stillbirth and shorter gestation.