The female breast is composed of milk glands, milk ducts, connective tissue, and fat. Because the breast is not composed of muscle, growth is influenced by a number of factors. Though girls begin to develop breasts at puberty, factors such as weight gain, age, pregnancy, menstruation, and menopause play a role in the size and appearance of breasts during a woman's life. It is important that a woman understand the anatomy and signs of development of her breasts in order to recognise any unhealthy abnormalities as soon as they appear.
During puberty, the ovaries begin to secrete the hormones oestrogen and progesterone, which allow the connective tissue in the breast to accumulate fat. During this time, the milk-duct system also begins to grow, increasing the size of the breasts. For many young girls, this initial development of breast tissue can be painful. Menstruation typically begins one to two years after breast development has begun, and may also cause a short-term increase in breast size. Hormonal changes during menstruation cause the breasts to temporarily swell. The milk glands also enlarge during menstruation, causing the breasts to retain water, which often results in pain or tenderness; these symptoms dissipate in the week following menstruation. During pregnancy, the breasts and milk-duct system both grow rapidly, causing tenderness on the sides of the breasts and sore or tingly nipples. Any time the breasts grow, the skin stretches, which can cause mild itching.
Other than the obvious increase in size, growing breasts often show a number of other physical characteristics. As breasts develop during puberty, the areolae become flat and only the nipple rises from the breast. During both puberty and pregnancy, breast growth can occur very rapidly, causing small tears in the tissue underneath the skin. This tearing results in long, finger-like pink or red scars known as stretch marks, which usually fade over time. During pregnancy, blood vessels are prominent on the breast owing to increased blood flow, and the areolae often darken and may grow larger in anticipation of breastfeeding.
Whether the result of puberty, menstruation, or pregnancy, breast size most often increases owing to an increased production of hormones. This fluctuation of hormones often results in mood swings, especially since oestrogen production is linked to the production of the "feel-good" chemical serotonin. Some medical professionals believe that there may be a link between oestrogen levels and depression, but in most cases, emotional disturbances are mild.