From the fourth month of pregnancy (the second and third trimesters), a woman's body produces hormones that stimulate the growth of the milk duct system in the breasts:
Progesterone — influences the growth in size of alveoli and lobes. Progesterone levels drop after birth. This triggers the onset of copious milk production.
oestrogen — stimulates the milk duct system to grow and become specific. Oestrogen levels also drop at delivery and remain low for the first several months of breastfeeding. It is recommended that breastfeeding mothers avoid oestrogen-based birth control methods, as a spike in estrogen levels may reduce a mother's milk supply.
Follicle stimulating hormone (FSH)
Luteinizing hormone (LH)
Prolactin — contributes to the increased growth of the alveoli during pregnancy.
Oxytocin — contracts the smooth muscle of the uterus during and after birth, and during orgasm. After birth, oxytocin contracts the smooth muscle layer of band-like cells surrounding the alveoli to squeeze the newly-produced milk into the duct system. Oxytocin is necessary for the milk ejection reflex, or let-down to occur.
Human placental lactogen (HPL) — From the second month of pregnancy, the placenta releases large amounts of HPL. This hormone appears to be instrumental in breast, nipple, and areola growth before birth.
By the fifth or sixth month of pregnancy, the breasts are ready to produce milk. It is also possible to induce lactation without pregnancy.
During the latter part of pregnancy, the woman's breasts enter into the Lactogenesis I stage. This is when the breasts make colostrum (see below), a thick, sometimes yellowish fluid. At this stage, high levels of progesterone inhibit most milk production. It is not a medical concern if a pregnant woman leaks any colostrum before her baby's birth, nor is it an indication of future milk production.
At birth, prolactin levels remain high, while the delivery of the placenta results in a sudden drop in progesterone, estrogen, and HPL levels. This abrupt withdrawal of progesterone in the presence of high prolactin levels stimulates the copious milk production of Lactogenesis II.
When the breast is stimulated, prolactin levels in the blood rise, peak in about 45 minutes, and return to the pre-breastfeeding state about three hours later. The release of prolactin triggers the cells in the alveoli to make milk. Prolactin also transfers to the breast milk. Some research indicates that prolactin in milk is higher at times of higher milk production, and lower when breasts are fuller, and that the highest levels tend to occur between 2 a.m. and 6 a.m.
Other hormones—notably insulin, thyroxine, and cortisol—are also involved, but their roles are not yet well understood. Although biochemical markers indicate that Lactogenesis II begins about 30–40 hours after birth, mothers do not typically begin feeling increased breast fullness (the sensation of milk "coming in") until 50–73 hours (2–3 days) after birth.
Colostrum is the first milk a breastfed baby receives. It contains higher amounts of white blood cells and antibodies than mature milk, and is especially high in immunoglobulin A (IgA), which coats the lining of the baby's immature intestines, and helps to prevent germs from invading the baby's system. Secretory IgA also helps prevent food allergies. Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk.
The hormonal endocrine control system drives milk production during pregnancy and the first few days after the birth. When the milk supply is more firmly established, autocrine (or local) control system begins. This stage is called Lactogenesis III
During this stage, the more that milk is removed from the breasts, the more the breast will produce milk. Research also suggests that draining the breasts more fully also increases the rate of milk production. Thus the milk supply is strongly influenced by how often the baby feeds and how well it is able to transfer milk from the breast. Low supply can often be traced to:
1.) not feeding or pumping often enough
2.) inability of the infant to transfer milk effectively caused by, among other things:
3.) jaw or mouth structure deficits
4.) poor latching technique
5.) rare maternal endocrine disorders
6.) hypoplastic breast tissue
7.) a metabolic or digestive inability in the infant, making it unable to digest the milk it receives
inadequate calorie intake or malnutrition of the mother
Milk ejection reflex
The release of the hormone oxytocin leads to the milk ejection or let-down reflex. Oxytocin stimulates the muscles surrounding the breast to squeeze out the milk. Breastfeeding mothers describe the sensation differently. Some feel a slight tingling, others feel immense amounts of pressure or slight pain/discomfort, and still others do not feel anything different.
The let-down reflex is not always consistent, especially at first. The thought of breastfeeding or the sound of any baby can stimulate this reflex, causing unwanted leakage, or both breasts may give out milk when an infant is feeding from one breast. However, this and other problems often settle after two weeks of feeding. Stress or anxiety can cause difficulties with breastfeeding.
A poor milk ejection reflex can be due to sore or cracked nipples, separation from the infant, a history of breast surgery, or tissue damage from prior breast trauma. If a mother has trouble breastfeeding, different methods of assisting the milk ejection reflex may help. These include feeding in a familiar and comfortable location, massage of the breast or back, or warming the breast with a cloth or shower.
The surge of oxytocin that triggers the milk ejection reflex also causes the uterus to contract. During breastfeeding, mothers may feel these contractions as afterpains. These may range from period-like cramps to strong labour-like contractions and can be more severe with second and subsequent babies.
Lactation without pregnancy
Women who have never been pregnant are sometimes able to induce enough lactation to breastfeed. This is called "induced lactation". A woman who has breastfed before and re-starts is said to "relactate". If the nipples are consistently stimulated by a breast pump or actual suckling, the breasts will eventually begin to produce enough milk to begin feeding a baby. Once established, lactation adjusts to demand. This is how some adoptive mothers, usually beginning with a supplemental nursing system or some other form of supplementation, can breastfeed. There is thought to be little or no difference in milk composition whether lactation is induced or a result of pregnancy. Rare accounts of male lactation (as distinct from galactorrhea) exist in medical literature.
Some drugs, primarily atypical antipsychotics such as Risperdal, may cause lactation in both women and men. Also, some couples may use lactation for sexual purposes.