An overview of the human mammary gland with a focus on the role of breast stem cells during pregnancy. The primary function of the mammary gland is to produce milk to nourish young offspring. The mammary gland is comprised of three main cell types; alveolar, ductal and myoepithelial cells. During pregnancy, the mammary gland increases in size due to the action of breast stem cells, which can mature into any of the three mammary gland cell types.