This is the first study to use US-wide data to explore how influenza exposure affects fetal development. I regress individual-level birth outcomes on monthly county-level influenza mortality rates, controlling for an extensive set of county- and time fixed effects. My approach isolates variation in influenza exposure arising from larger-than-average epidemics and minimizes confounding due to factors that vary by county, season, and year. Exposure to severe influenza epidemics during critical gestation periods can increase neonatal and infant mortality rates by more than 45 percent, decrease birthweight by 44 grams, increase the probability of low birth weight by 15 percent, and increase the risk of preterm births by 16 percent. The first trimester is a critical period for mortality and preterm birth, while the third trimester is a critical period for preterm birth, gestational age, and birthweight. Differential exposure to influenza may be a driver of broader seasonal patterns in birth outcomes.
Published in a peer-reviewed journal of the Population Council.