Breasts have a role in sexual arousal, but their main function is to produce and deliver milk to a newborn baby. Hormonal changes in late pregnancy stimulate the production of milk in glands called lobules, which lead to ducts that converge and then open on to the surface of the nipple. The remaining breast tissue is mostly fat, with a small amount of connective tissue that helps to support the breasts. The variety in size and shape of breasts is determined by genetic factors, fat content, and muscle tone.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.