Menstruation and Menopause
Estrogen and progesterone cause the growth of milk ducts and stimulation of these milk ducts respectively. The former is increased in the first phase of the menstrual cycle, while the latter is at peak on the second phase of the cycle.
However, during the menopausal stage, the ovaries stop producing these two hormones. Consequently, this causes the breast to stop undergoing the cycle of growing and being stimulated.
Pregnancy and Lactation
Primarily, breasts are for milk production, storage, and release to feed an offspring. At the sixth month of pregnancy, mammary glands start to develop in preparation for lactation. After childbirth, female hormones such as prolactin and oxytocin stimulate mammary glands for increased milk production and release.
During breastfeeding, with the sucking motion of the baby, milk is delivered from the production glands to the nipples via the ducts. Milk is then released from the breasts through the nipples.
Since milk production is active during these stages, the breasts become bigger and fuller. However, once the child is weaned from breastfeeding, the breasts convert most mammary glands back to fats and ducts.
How Does Breast Cancer Develop?
Most breast cancers start in the upper outer quadrant of the breast, close to the armpit, where numerous glandular tissues are found.
Cancers start with healthy cells that become monstrous in nature, growing uncontrollably to form a tumor. When a tumor is growing but not spreading, it is benign by nature. However, when it starts to spread to other parts of the body, it becomes cancerous in nature. Metastasis occurs when breast cancer cells transfer to other parts of the body by the circulatory or lymphatic systems.