Breastfeeding is the process of a woman feeding an infant or young child with milk from her breasts. Babies have a sucking reflex that enables them to suck and swallow milk.

Also important in the process is an effective latch, a normal frenulum, and an adequate milk supply. For the equivalent in other mammals, see suckling.

Experimental evidence suggests that, barring certain health issues, human breast milk provides the optimal nutrition source for human infants. Disagreement does remain between experts regarding the optimal duration of breastfeeding to realize the benefits, as well as the relative risk of harm in using breast milk substitutes.

Breastfeeding may occur between the infant and its own mother, or another lactating female. Breast milk substitutes are available for mothers or families who cannot or prefer not to breastfeed their children. Examples of medically accepted alternatives to breastfeeding include feeding the infant expressed breast milk from its own mother, from another lactating female, pasteurized donor human milk, or commercially-available infant formulas. There are conflicting studies concerning the equivalence between available breast milk substitutes. In both term and preterm infants, the use of commercial breast milk substitutes have been proven safe and effective as a nutrition source but inferior to breastfeeding. Donor breast milk handling processes have been suspected in the reduction of effectiveness in pasteurized donor human milk.

Many governmental strategies and international initiatives have promoted breastfeeding as the best method of feeding a child in his or her first year and beyond, as does the World Health Organization (WHO) and the American Academy of Pediatrics (AAP).

Hormonal influences

Throughout the last two trimesters of pregnancy a woman's body produces hormones which stimulate the growth of the milk duct system in the breasts:

• Progesterone — influences the growth in size of alveoli and lobes. Progesterone levels drop along with estrogen levels after birth, triggering the onset of copious milk secretion
• Estrogen — stimulates the ductule system to grow and become specific. Estrogen levels drop at delivery and remain low for the first several months of breastfeeding. (This is also why it is recommended that breastfeeding mothers avoid estrogen-based birth control methods while they are planning to breastfeed. A spike in estrogen levels compromises a mother's milk supply level.)
• Follicle stimulating hormone (FSH)
• Luteinizing hormone (LH)
• Prolactin — contributes to the accelerated growth of the alveoli during pregnancy
• Oxytocin — contracts the smooth muscle of the uterus during birth, 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)—released in large amounts by the placenta during pregnancy (beginning in the second month); appears to be instrumental in breast, nipple, and areola growth before birth

By the fifth or sixth month of pregnancy, the breasts are sufficiently developed to produce milk. It is also possible to induce lactation.

During the latter part of pregnancy, the woman's breasts enter into the Lactogenesis I stage, where the breasts are making colostrum, a thick, sometimes yellowish fluid, as high levels of progesterone inhibit most milk secretion. It is considered medically normal for a pregnant woman to leak colostrum before her baby's birth, or not to leak at all. Neither situation is an indicator of future milk production levels in the mother.

At birth, 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 Lactogenesis II, copious milk production.

Prolactin blood levels rise when the breast is stimulated, peaking about 45 minutes later. Levels return to the pre-breastfeeding state about three hours afterwards. Prolactin release triggers the cells in the alveoli to create milk. Some research indicates that prolactin in milk is higher at times of higher milk production, 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 commences approximately 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.

The colostrum is the first milk a baby breastfeeding from birth 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, helping to prevent germs from invading the baby's system. Secretory IgA also works to help prevent food allergies. After a baby has been nursing for 3–4 days, the colostrum in the breast slowly begins the process of changing into mature breast milk over the next two weeks.

During pregnancy and the first few days postpartum, milk supply is hormonally driven. This is the endocrine control system. After milk supply has been more firmly established, Lactogenesis III begins - the autocrine (or local) control system.

At this stage, the more milk removed from the breast, the more milk the breast will produce. Thus milk supply is strongly influenced by how often the baby feeds and how well it is able to transfer milk out of the breast. Low supply can often be traced to:

• not feeding or pumping often enough
• inability of the infant to transfer milk effectively caused by, among other things:
  • jaw or mouth structure deficits
  • poor latching technique
• rare maternal endocrine disorders
• hypoplastic breast tissue
• a metabolic or digestive inability in the infant, rendering it unable to utilize the milk it receives
• inadequate calorie intake or malnutrition of the mother

Research on mothers who express their milk indicates that for most women the more times per day a mother expresses her milk, the more milk she produces. Research also suggests that more fully draining the breasts also increases the rate of milk production.

Feeding at least once every two to three hours helps to maintain the milk supply. For most women, a target of eight nursing sessions/pumping sessions per 24 hours seems to keep a milk supply high not only during the early months of lactation, but especially past the fourth month. It is not at all uncommon for newborn infants to nurse far in excess of this amount: 10 to 12 nursing sessions per 24 hours is the comparative norm, while some may even nurse 18 times in the same time frame. Feeding a baby on demand (sometimes referred to as "on cue"), which may mean nursing many times more than the recommended minimum, is the best way to maintain milk production and ensure the baby's needs for milk and comfort are being met satisfactorily. However, it may be important to recognize whether a baby is truly hungry, as breastfeeding too frequently may mean the child receives a disproportunately high amount of foremilk, and not enough hindmilk, potentially creating problems.

Milk ejection reflex

The milk ejection reflex (let-down reflex) is caused by the release of the hormone oxytocin. Oxytocin stimulates the muscles surrounding the breast to squeeze out the milk. Breastfeeding mothers describe the sensation differently, with some feeling a slight tingling, some feeling immense amounts of pressure, some feeling slight pain/discomfort, and still others not feeling anything different.

The reflex is not always consistent, especially intially. The thought of nursing or the sound of any baby can stimulate the milk ejection reflex, causing unwanted leakage, or both breasts giving out milk when one infant is feeding. However, this and other problems often settle after two weeks of feeding. If the mother is in a stressed or anxious state of mind this can cause difficulties with breastfeeding.

Causes of a poor milk ejection reflex can include sore or cracked nipples, separation from the infant, or a history of breast surgery. If a mother has trouble breastfeeding she can try different methods of assisting the milk ejection reflex. These include feeding in a familiar and comfortable location, massage of the breast or back, or warming the breast with a cloth or shower.

Afterpains

The surge of oxytocin triggering the milk ejection reflex also causes the uterus to subinvolute or contract. Subsequently, during breastfeeding mothers can feel uterine contractions (pain ranging from period-like cramps to strong labour-like contractions). Afterpains can be more severe with second and subsequent babies.

Lactation without pregnancy

Although it is not widely known in developed countries, women who have never been pregnant are sometimes able to stimulate lactation sufficient to breastfeed. This is called "induced lactation", while a woman who has lactated before and re-starts is said to "relactate". If the nipples are stimulated as in breastfeeding for a while (such as by a breast pump or actual suckling), eventually the breasts will begin to produce milk which can be used to feed a baby. Once established, lactation adjusts to demand. For this reason, adoptive mothers, usually initially in conjunction with some form of supplementation, such as a supplemental nursing system, are able to breastfeed their infants and young children. 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 the medical literature.

Some couples may choose to induce lactation as a sexual practice; see Erotic lactation.

Additionally, some drugs, primarily atypical antipsychotics such as Risperdal, may cause lactation in both women and men.

Breast milk

The exact properties of breast milk are not entirely understood, but the nutrient content of mature milk is relatively consistent and draws its ingredients from the mother's food supply and the nutrients in her bloodstream at the time of feeding. If that supply is inadequate, content is obtained from the mother's bodily stores. (Some studies estimate that a woman exclusively nursing her infant uses an extra 500–600 calories per day simply producing milk for her offspring.) The exact composition of breast milk varies from day to day, and even hour to hour, depending on both the manner in which the baby nurses and the mother's food consumption and environment, so the ratio of water to fat fluctuates.

Foremilk, the milk released at the beginning of a feed, is watery, low in fat and high in carbohydrates compared with the creamier hindmilk which is increasingly released as the feed progresses. There is no sharp distinction between foremilk and hindmilk – the change is very gradual. Research from Peter Hartmann's group tells us that fat content of the milk is primarily determined by the emptiness of the breast—the less milk in the breast, the higher the fat content. The breast can never be truly "emptied" since milk production is continuous.