The introduction of the first antibiotics in the United States in 1937 led to sharp drops in infant mortality from pneumonia and maternal mortality from puerperal sepsis. We leverage this plausibly exogenous medical innovation to examine fertility responses among women giving birth in this era. The decline in pneumonia mortality led women to have fewer children overall, with fewer women having three or more children, and a larger share remaining childless. We present a new theory of the extensive margin response in terms of infant mortality decline decreasing the years women need to achieve their target number of births. This prompts fertility delay and labor market entry which, coupled with wage or fecundity shocks can result in childlessness. We show that reductions in infant mortality increased women’s labor force participation, improved their occupational status and reduced their chances of ever having married. Maternal mortality created opposing effects on all of these outcomes. Joint with Atheendar Venkataramani and Selma Walther.