We examine the effect of a Chinese family planning policy (FPP) known as "Later, Longer, and Fewer" on the gender gap in breastfeeding. We find that FPP increased the daughterson breastfeeding gap in favor of sons in rural areas. Mean intensity of the FPP predicts the gender gap to be 35% greater than the gap without FPP. The effects are explained by the skewed gender composition of last-born children produced by sex-selective stopping behavior. The findings indicate a way in which FPP, in the context of son preference, widens gender gap in child development.