Two of the body’s natural hormones, prolactin and oxytocin, play a vital role in nursing. Prolactin is used to motivate the cells to being milk production. This hormone is released slowly during pregnancy and increases during nursing. The longer a baby feeds, the more prolactin is released. Oxytocin then causes the breast cells to contract, assisting the milk in eject. This is called the letdown reflex.
These hormones begin circulation as soon as a mother becomes pregnant. By the second trimester, your body may begin secreting some of its first milk, colostrum. This is when the breasts are fully capable of producing milk. However, this secretion will be minor until childbirth when the colostrum begins to letdown in earnest.
Occasionally, milk production can occur outside of pregnancy but this is generally because of a hormone imbalance, a pituitary tumor, or the regular stimulation of your nipples.