Breast milk

Alternative names 
Milk - human; Human milk


Breast milk is the perfect source of nutrition for infants. Breast milk contains appropriate amounts of carbohydrate, protein, and fat. It also provides digestive enzymes, minerals, vitamins, and hormones that all infants require. Breast milk contains valuable antibodies from the mother that can help the baby resist infections.

Healthy infants have adequate iron stores to last until 8 months of age. Iron-rich foods can be started at this age. (See diet for age.) Your pediatrician or dietitian may recommend fluoride supplementation in communities where water is not fluoridated (areas with less than 0.25 p.p.m. fluoride).

Cow’s milk by itself is inappropriate for infants less than 1 year old. The infant can develop an allergy to dairy products if given cow’s milk too early in life. Although cow’s milk contains most of the same components as breast milk, these components are not in the same amounts. Cow’s milk also lacks the immune factors, called antibodies, that help protect infants until their own immune system fully develops.

Commercially prepared formulas may be based on non-fat cow’s milk, whey protein, or soy protein. In order to provide a balanced diet for an infant, formulas must be fortified with carbohydrates, fats, minerals, and vitamins. The antibodies found in breast milk, however, can never be added to formulas.


Milk is produced in small sac-like glands in the breast. These sacs develop after specific hormones (such as estrogen, progesterone, pituitary prolactin, and placental lactogen) stimulate them, beginning during the second trimester of pregnancy.

The human breast does not store a large volume of milk, as cows do. Suckling stimulates the release of a hormone (prolactin) which stimulates milk production and the release of another hormone (oxytocin). Oxytocin, in turn, stimulates contraction (or the “let-down reflex”) of the milk glands. The milk is squeezed out of the milk gland, into the milk ducts, and into the nipple.

At the beginning of the feeding, the milk is bluish and contains lactose and proteins, but little fat; it is called foremilk. The end of the feeding produces hindmilk. The hindmilk contains more fat, the main source of energy for your baby. If breast milk is allowed to sit for half-an-hour after being expressed, the “cream” separates and settles on top of the watery part. This is because human milk isn’t homogenized, the process that makes the water and fat portion in milk stay in “one layer.”


Some mothers feel a tingling sensation as the milk begins to eject from the breast due to the let-down reflex. The best way to monitor whether this milk-ejection reflex has begun is by watching the sucking and breathing patterns of your baby.

At the start of a feeding, you will notice a pattern such as: suck, suck, suck, swallow, suck, suck, suck, swallow. This pattern may last up to 30 seconds, but could be longer. As the milk ejection reflex takes over, the pattern may be described as gulping: one swallow, one deep breath, one swallow, one deep breath. This pattern may last for 2 to 4 minutes.

Your baby may want to nurse longer (maybe 15 or 20 minutes) on this breast in order to get more fat from the hindmilk or because your baby needs to fill a need for sucking. Rather than watching the clock to let him nurse “10 minutes on each side,” you may want to allow your baby to break off the first breast at his own timing before switching to the other breast.


Your milk supply will be established during the first few days and weeks after the birth of your baby. Nursing early (within the first half-hour), and frequently (on demand, or 8-12 times per day), allows you to nurse comfortably and efficiently. It usually takes less than one minute for an infant to stimulate the milk ejection reflex. You should feel little discomfort or pain when breast feeding appropriately.

Within 6 and 8 weeks, your milk supply will adjust to your baby’s needs. Before that time, your breasts may feel either too full or too empty. Frequent, comfortable feedings will maintain your milk supply. Your milk supply will increase or decrease based on your baby’s hunger and energetic sucking (milk demand or use). Changes in your milk supply will occur within one to three days after changes in milk demand or use.


When storing milk for home use, wash your hands before expressing (or pumping). Use containers that have been washed in hot, soapy water and rinsed well. Always date the milk before storing it.

Fresh breast milk can be kept at room temperature up to 10 hours, and refrigerated up to 1 week. Frozen milk can be kept in a freezer compartment inside the refrigerator for 2 weeks; in a separate door refrigerator/freezer up to 3 or 4 months; and in a deep freezer at constant 0 degrees for 6 months. Frozen and thawed milk can be refrigerated for up to 9 hours, but it should not be refrozen.

Plastic containers are the best for storing breast milk. For freezing, use small (2 or 3 ounce) containers to avoid the waste of unused portions at the end of the day. Refrigerated milk and frozen milk should be warmed under a stream of warm tap water. Never microwave breast milk (overheating destroys valuable nutrients and “hot spots” can scald your baby).

Johns Hopkins patient information

Last revised: December 8, 2012
by Brenda A. Kuper, M.D.

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.