There are two mechanisms that are in place for the production of breast milk. The rapid drop in hormones that occurs after delivery of the baby is responsible for the initiation of the breast milk. This takes 2-3 days after delivery. The potentiation or continuation of the breast milk is caused by a hormone called prolactin. Prolactin is produced by the brain in response to suckling and is responsible for the breast to continue to make milk.
The rapid fall in hormones after delivery causes the breast milk to come in. During the 2-3 days previous to the milk coming in colostrum is produced. Colostrum is mostly serum and antibodies, and is what the baby needs for the first couple of days. The new mother will experience this rapid hormone drop whether or not she is breast feeding. This is why the milk coming in is not dependent on suckling and will take place even in the absence of a breast feeding baby.
When the baby suckles on the breast it sends a signal to the brain. The brain produces a hormone called prolactin in response to this signal. Prolactin acts on the breast to stimulate the production of milk. The breasts work on a supply and demand basis. The more the baby suckles the more prolactin is made and the more milk that is produced. From time to time the baby will go through a growth spurt and will suddenly want more milk. It takes the body about 24 hours to produce more milk in response to the increased demand. It is not uncommon for the baby to get fussy for about a day when a growth spurt happens. Babies can be impatient and not enjoy the 24 hour wait for more milk.
The breasts are capable of supplying a very large quantity of milk in response to increasing demand. The human body is capable of producing enough milk to feed at least 4 babies. However it is difficult to produce this much milk because of the amount of time babies will have to be suckling. Do not worry if you think you can not make enough milk to keep up with your hungry baby, you can.
Prolactin also inhibits the ovaries form ovulating and producing estrogen. Women that exclusively breast feed get some contraceptive benefit. Women that breast feed on demand get more ovarian inhibition than women that breast feed on a schedule. This is due to the increase frequency of prolactin production in the “on demand” situation. Even though the overall amount of prolactin is similar the increased frequency of prolactin production inhibits the ovaries more efficiently. In women who breast feed on demand and exclusively, there is a 90% contraception benefit. Because of the ovarian inhibition, breast feeding women can have issues with decreased vaginal lubrication. If this is an issue, lubricants can be used to help with vaginal dryness.