Breastfeeding: How the Breasts Function

Most mothers are physically capable of nursing their babies successfully. In fact, physical inability to breast feed is extremely rare. A mother’s capacity to nurse her baby is not particularly affected by her age, her diet, how many children she has nursed, the season of the year, the return of menstruation or the size of her breasts.

The first substance in the breast available to the newborn baby has a special name, “colostrum”. It is actually the forerunner of breast milk. Analgesia and anesthesia used for labor and delivery may sometimes delay the “coming-in” of breast milk, but the effect is temporary.

Colostrum and breast milk are very special, for they contain factors which can help to protect the baby from disease, infection, allergy and tooth decay. Science has never been able to duplicate colostrum or breast milk. Although many of their components have been duplicated, many other important ones have not. Even those components which have been reproduced have not been combined in a way which duplicated colostrum or breast milk.

Colostrum and breast milk are made by tiny glands in the deepest parts of the breasts. These glands (alveoli) grow larger and more numerous during pregnancy. After birth the level of hormones (estrogen and progesterone) in the mother’s blood begin to drop. As a result, these glands begin to make milk from various components of the blood. Two or three days later, sometimes longer, the milk from these glands reaches the milk sinuses through small ducts. The milk sinuses, which lie under the dark area (areola) around the nipple, act as reservoirs. The breast milk which collects in these milk sinuses between feedings is called “fore milk” and is the first milk available to the baby as he begins each feeding. Fore milk is high in protein but low in fat.

Once the baby begins to suckle, the stimulation sends a signal to an area of the brain (hypothalamus). The pituitary gland at the base of the brain is then stimulated to release the hormone oxytocin into the blood stream. The oxytocin reaches the breast and causes the band-like (myoepithelial) cells around the alveoli to contract, squeezing out the breast milk into the duct system, where it becomes available to the baby through the nipple openings. This squeezing out action (ejection mechanism) is called the “let-down” and is noticeable when the baby pulls away from the breast and a jet of milk is observed spraying from the nipple. Sometimes, when the mother just thinks about her baby, let-down will occur. Embarrassment, pain, fear or anger may temporarily inhibit the let-down reflex. Let-downs will occur repeatedly during a feeding, as the infant follows the normal eat-sleep-eat pattern of the newborn.

When let-down occurs, the deeper milk called “hind milk” becomes available to the baby. It is relatively high in fat content (4 to 7 percent) and makes up two thirds of the breast milk available to the baby, if the baby is allowed enough time to empty the breast. Actually, the breast is never really empty, since the production of breast milk is more or less continuous, but after letdown has occurred, few babies get much milk from the breast after 10 minutes of nursing.

Quantity of Milk

The quantity of milk produced by the breast depends greatly on how often and how vigorously the breast is suckled. Mothers frequently breast feed twins without supplemental formulas. The more sucking stimulation the breasts receive, the more milk they produce. (There is usually a delay of about 48 hours between an infant’s increased demand and a corresponding increase in the mother’s milk supply.) By the same token, the less stimulation the breasts receive, the less milk they produce. If the baby’s appetite is blunted by solid food or supplemental formula or if he is not allowed to nurse often or long enough, the breasts will produce less and less milk.

Quality of Milk

The composition and quality of breast milk is relatively unaffected by the mother’s intake of fluids or her diet. Nature protects the infant by causing the mother to become thirsty before her milk supply is affected. If she drinks more fluids than she needs, she merely increases her output of urine. The concentration of fat and vitamins in breast milk is somewhat affected by maternal diet, but on the whole the composition of breast milk seems to be an individual characteristic of the mother. Harfouche, in his 1967 report to the World Health Organization on “The importance of Breastfeeding”, stressed that the composition of human milk is largely dependent on the state of nutrition of the mother.

Avoiding Difficulties

Some discomfort should be expected in getting started at breastfeeding, but much unnecessary discomfort can be avoided if the mother nurses as soon after birth as possible. By nursing soon after birth and nursing at least every three hours during the day, and at least once at night, the infant can clear the relatively thick colostrum from the breast ducts which carry the milk to the nipple pores. If the duct system is not sufficiently cleared of colostrum before the milk begins to accumulate, the back pressure caused by the collecting milk starts a chain reaction, which results in unnecessary discomfort for the mother and frustration for her baby, as the breasts become overly firm and the areola and the nipple become difficult or impossible for him to take into his mouth.

Most breast infections are preventable. Mothers should avoid going too long between feedings or upsetting the normal feeding pattern, especially in the beginning. If, for any reason, a mother misses a feeding, she should empty her breasts of milk by manual expression or with a breast pump as close to the time she would normally nurse her baby as is possible. If the breasts are not emptied regularly, milk may plug a small duct in the breast, causing a reddened, tender spot to develop. If this should happen, the mother should nurse more frequently from the affected breast in order to clear the duct (sudden weaning may result in a persistent infection), apply moist heat to the affected area, rest, drink more fluids and call your doctor or certified nurse midwife. A nursing mother should check with her pediatrician before taking any medicine.


Milk leaking from the breast is usually only temporary and is more apt to occur when a mother is nursing for the first time. Leaking usually stops or can be controlled once the breasts have adjusted their production of milk to the needs of the infant and the walls of the milk sinuses have become more elastic and can expand enough to store the milk produced by the breast. However, from time to time an emotional reaction may cause an unexpected let-down of milk. Fortunately the let-down reflex is often preceded by a tingling sensation in the nipple and areola areas. Pressure, applied by the mother, to these areas of both breasts when the tingling first begins, is usually sufficient to prevent leaking.

Does Breastfeeding Effect My Period?

As long as a mother’s breasts receive regular, strong stimulation from her baby’s vigorous sucking, the pituitary gland in the mother’s brain releases a hormone (prolactin) into her blood stream, preventing the ovary from releasing an egg (ovulating). The early introduction of solid food or supplemental formula may result in the mother’s breasts receiving less and less stimulation. Consequently, as the prolactin level in the mother’s blood drops, the ovary is more likely to release an egg, making conception possible even before menstruation occurs. Menstruation does not affect the quality or quantity of breast milk.


Several factors work together to make weaning, when properly carried out, a mutually comfortable experience for both mother and baby, although most mothers express some regret at losing this special bond. As the baby’s appetite is blunted by solid foods and supplemental fluids, he suckles the breast less frequently and with less vigor. This decreased stimulation causes the production of breast milk to diminish, making less milk available to the infant. The baby becomes less willing to put forth the effort of suckling when he can so easily suck cow’s milk from a bottle or drink from a cup. Any milk which remains in the breasts after the mother stops breastfeeding is gradually absorbed by the body.

How does a baby get milk from the breast?

The oral mechanics of “suckling” from the breast and “suckling” from a rubber nipple are quite different. This is why offering a newborn infant fluids, even breast milk, from a bottle, tends to confuse and weaken the infant’s ability to suckle.

An infant suckles milk from his mother’s breasts by the compression of his gums and by suction. As the infant begins to suckle, he opens his mouth and thrusts his tongue forward to grasp the nipple and areola from underneath. As his tongue pulls backward, the nipple is brought up against the hard palate of his mouth. Suction, created by negative mouth pressure, keeps the nipple in place, while the infant’s jaw action causes his gums to compress the milk sinuses beneath the areola, squeezing out the milk into the back of his throat. Reflex action causes the baby to swallow when enough milk has been collected. If, for any reason, an infant cannot suckle effectively or must remain in the nursery, an electric breast pump, which is quite comfortable to use, is usually available from the hospital or a medical supply store. The mother’s milk is then given to her baby by bottle, if her pediatrician so orders.

How can I tell if my breasts are making enough milk?

If the nursing baby has six or more wet diapers a day, or pale urine, and is gaining weight, even if slowly, this usually indicates that the mother is producing enough breast milk. Of course, if the baby is getting formula, water or juice from a bottle, this is not a good way to judge. Testing the adequacy of the mother’s supply of breast milk by offering the baby a bottle is not a reliable guide, since a baby’s nursing reflex is so automatic that he will usually take one or two ounces of formula from a bottle even when he has been adequately fed at the breast.