Hyperprolific sows are required to produce more milk to achieve good piglet growth rate, and one of the determinants of milk yield is mammary development. Mammary development occurs during 3 months of age until puberty, the end third of gestation, and lactation. Estrogen, prolactin, and relaxin are the hormones involved for gestational development, and prolactin injections given to gilts stimulated mammary development after puberty. Restricting gilt feed at 90 days until puberty reduced mammary development, but reduced protein during that time or restricting feed from day 28-90 had no effect. Feeding genistein or 10% flaxseed increased mammary development. Gestational diets that are high-energy low-protein reduce mammary development, but high protein diets can increase milk yield. lactation diets should be high in both energy and protein for high milk yield. As well, teats that are suckled during the first lactation will produce more milk for the second, which can be used to decide how many pigs to put on primiparous sows.