Also known as the mammary glands, the breasts are glands that develop on the chest wall of women at puberty. Some women have breasts that are higher or lower on the chest, but when kneeling on all fours so the breast is hanging down, the nipple is usually over the fourth to sixth rib on each side. Some women have round breasts, while others have a more tubular shape. The size, shape and position of the breast is determined genetically, so women are likely to have a similar shaped and sized breasts to that of their mother and both maternal and paternal grandmothers.
The milk glands are arranged into 15 to 20 groups (lobes), each of which drains separately through ducts in the nipple. The amount of milk producing glandular tissue is similar in all breasts, regardless of their size. Larger breasts merely have more fat in them.
During pregnancy the glandular tissue increases to enlarge the breasts, and make them tender at times. The same phenomenon occurs to a minor extent just before a period in many women due to the increased level of oestrogen (sex hormone produced by the ovaries) in the bloodstream.
The breast also contains fibrous tissue to give it some support. The stretching of these fibres causes the breast to sag after breastfeeding and with age.
When stimulated by suckling, muscles in the nipple contract to harden and enlarge it so that the baby can grip and suck on it. A similar response occurs with sexual activity, cold or emotional excitement.