Medical Encyclopedia


Medical Encyclopedia

Other encyclopedia topics:  A-Ag  Ah-Ap  Aq-Az  B-Bk  Bl-Bz  C-Cg  Ch-Co  Cp-Cz  D-Di  Dj-Dz  E-Ep  Eq-Ez  F  G  H-Hf  Hg-Hz  I-In  Io-Iz  J  K  L-Ln  Lo-Lz  M-Mf  Mg-Mz  N  O  P-Pl  Pm-Pz  Q  R  S-Sh  Si-Sp  Sq-Sz  T-Tn  To-Tz  U  V  W  X  Y  Z  0-9 

Breast milk

Contents of this page:

Alternative Names   

Milk - human; Human milk; Milk - breast

Function    Return to top

Breast milk is the perfect source of nutrition for infants. Breast milk contains appropriate amounts of carbohydrate, protein, and fat. It also provides the digestive proteins, minerals, vitamins, and hormones that infants need. Breast milk contains valuable antibodies from the mother that may 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. Your pediatrician or dietitian may recommend fluoride supplementation if you live in a community where water is not fluoridated.

Cow's milk by itself is inappropriate for infants less than 1 year old. The infant may 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 (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 called prolactin, which stimulates milk production and the release of another hormone called oxytocin. Oxytocin 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. Such milk 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 blended.


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: 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 - 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 pull away from the first breast naturally before switching to the other breast.

Recommendations    Return to top

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 1 minute for an infant to stimulate the milk ejection reflex. You should feel little discomfort or pain when breastfeeding appropriately.

Within 6 - 8 weeks, your milk supply will adjust to your baby's needs. Before that time, your breasts may feel either too full or 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 1 - 3 days after changes in milk demand or use.


When storing milk for home use, wash your hands before expressing (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 8 hours, and refrigerated for 5 to 7 days. 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; or 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.

Related topics:

Update Date: 11/30/2008

Updated by: Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M. Logo

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. Copyright 1997-2009, A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.