- STUDY QUESTIONDoes maternal exposure during pregnancy to higher ambient temperature increase the risk of congenital heart defects (CHDs)?SUMMARY ANSWERSignificant associations were found between maternal exposure during pregnancy to higher ambient temperature and CHDs risk especially during the cold season.WHAT IS KNOWN ALREADYFrom rodents to non-human primates, a teratogenic effect of hyperthermic insult has been demonstrated. There are fewer data regarding the effect on the human fetus and specifically the association between maternal exposures during pregnancy to higher ambient temperature and CHDs.STUDY DESIGN, SIZE, DURATIONThis population registry-based cohort study included 135 527 live and stillbirths in the Tel-Aviv region of Israel in 2000-2006.PARTICIPANTS/MATERIALS, SETTING, METHODS Two clinical diagnostic groups of isolated cardiac defects (atrial septal defects and ventricular septal defects: n = 542 and 481, respectively) and one group of multiple cardiac defects (defined by the presence of two or more cardiac malformations, n = 607) were studied. Temperature measurements were constructed from ambient stations and used to assess the impact of maternal exposure to average ambient temperature and extreme heat events (daily average temperature above the 90th percentile) during Weeks 3-8 of pregnancy on risk of CHDs. Logistic models, adjusted for sociodemographic covariates, were used to evaluate the associations between temperature and CHDs.MAIN RESULTS AND THE ROLE OF CHANCEOverall, we found no significant associations between ambient temperature and CHDs throughout the year, with one exception for multiple CHDs. After stratifying by season of conception, continuous exposure to average ambient temperature and maximum peak temperature (1°C increase) during the cold season increased the risk for multiple CHDs [odds ratio (OR) 1.05, 95% confidence interval (CI): 1.00, 1.10 and OR 1.03, 95% CI: 1.01, 1.05, respectively]. A 1-day increase in extreme heat events increased the risk for multiple CHDs (OR 1.13, 95% CI: 1.06, 1.21) and also for isolated atrial septal defects (OR 1.10, 95% CI: 1.02, 1.19).LIMITATIONS, REASONS FOR CAUTIONInformation both on CHD cases and on ambient temperature was based on registries and it is possible that this may cause some misclassification. In urban areas, pregnant women may be exposed to higher temperatures than recorded by ambient monitors because of the 'heat island effect'. Furthermore, data for the amount of time spent indoors were unavailable and this could have resulted in exposure misclassification.WIDER IMPLICATIONS OF THE FINDINGSThe findings are important within the context of global climate change, which may have implications for public health in countries with mild winters and hot summers.STUDY FUNDING/COMPETING INTEREST(S)This study was supported by the Israeli Ministry of Environmental Protection (research grant-7-2-7) and by the Environment and Health Fund (PhD Fellowship Program). There are no competing interests.