"Medical research indicates that breastfeeding suppresses post-natal fertility. We model the implications for breastfeeding decisions and test the model's predictions using survey data from India. First, we find that breastfeeding increases with birth order, since mothers near or beyond their desired total fertility are more likely to make use of the contraceptive properties of nursing. Second, given a preference for having sons, mothers with no or few sons want to conceive again and thus limit their breastfeeding. We indeed find that daughters are weaned sooner than sons, and, moreover, for both sons and daughters, having few or no older brothers results in earlier weaning. Third, these gender effects peak as mothers approach their target family size, when their decision about future childbearing (and therefore breastfeeding) is highly marginal and most sensitive to considerations such as ideal sex composition. Because breastfeeding protects against water- and food-borne disease, our model also makes predictions regarding health outcomes. We find that child-mortality patterns mirror those of breastfeeding with respect to gender and its interactions with birth order and ideal family size. Our results suggest that the gender gap in breastfeeding explains 14 percent of excess female child mortality in India, or about 22,000 "missing girls" each year"--National Bureau of Economic Research web site.