Prevention of mother-to-child transmission (PMTCT) interventions reduce the cumulative probability of transmission from a HIV positive woman to her child by as much as 40 percentage points. This paper is the first economic analysis of the behavioral effects of PMTCT. The author examine fertility responses to the scale-up of PMTCT in Zambia, a country where approximately 15 percent of adults age 15-49 are HIV positive. The author's results suggest that the local introduction of PMTCT reduced pregnancy rates by up to 20 percent, that the fertility response was greater among women who were more likely to be HIV positive, and that PMTCT substantially increased breastfeeding rates.