The overall objective of this study is to examine the impact of climate change on the risk of preterm delivery (PTD) by using a case-crossover study design utilizing members of KPNC. In our first aim, we will assess the relationship between apparent temperature and PTD. Estimates will be adjusted for important air pollutants. In our second aim we will determine whether maternal factors (age, race/ethnicity and SES as measured by medicaid status) impact the relationship between apparent temperature and preterm delivery. Finally, we will explore whether additional maternal factors may impact the the relationships in the first aim. These additional factors include hypertension, smoking, alcohol use, medication use, gestational diabetes, depression, and pre-pregnancy BMI.