Prevention of mother-to-child transmission (PMTCT) interventions reduce the cumulative probability of a HIV positive woman transmitting the virus in utero, during childbirth, or through breastfeeding by as much as 50 percentage points. Between 2000 and the end of 2007, the number of health clinics in Zambia offering PMTCT increased from fewer than 6 to nearly 600. This paper examines fertility responses to the scale-up of PMTCT in Zambia, a country where more than 15 percent of adults age 15-49 are HIV positive. I use multiple rounds of nationally representative household survey data, newly assembled panel data on the expansion of HIV/AIDS health services, and detailed spatial data on the location of households and health facilities. My results indicate that the local introduction of PMTCT reduced pregnancy rates by approximately 20 percent, that the fertility response was greater among women more likely to be HIV positive, and that PMTCT increased breastfeeding rates.