Temperature and Term Low Birth Weight in California

: Few investigations have explored temperature and birth outcomes. In a retrospective cohort study, we examined apparent temperature, a combination of temperature and relative humidity, and term low birth weight (LBW) among 43,629 full-term LBW infants and 2,032,601 normal-weight infants in California (1999-2013). The California Department of Public Health provided birth certificate data, while meteorological data came from the California Irrigation Management Information System, US Environmental Protection Agency, and National Centers for Environmental Information. After considering several temperature metrics, we observed the best model fit for term LBW over the full gestation (per 10-degrees-Fahrenheit (°F) increase in apparent temperature, 13.0% change, 95% confidence interval: 4.1, 22.7) above 55°F, and the greatest association was for third-trimester exposure above 60°F (15.8%, 95% confidence interval: 5.0, 27.6). Apparent temperature during the first month of pregnancy exhibited no significant risk, while the first trimester had a significantly negative association, and second trimester, last month, and last 2 weeks had slightly increased risks. Mothers who were black or older, delivered male infants, or gave birth during the warm season had infants at the highest risks. This study provides further evidence for adverse birth outcomes from heat exposure for vulnerable subgroups of pregnant women.