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Breast Milk

A Definition and Description of Human Breast Milk


Updated July 05, 2014

Breast Milk
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What is breast milk?

Breast milk is a liquid produced from the breasts of a mother for her children. It is created in response to pregnancy and the suckling of a baby at the breast. Breast milk provides a child with complete nutrition, protection against infections, diseases and illnesses, and a multitude of other health and developmental benefits that continue long after breastfeeding has ended.

The composition of breast milk is complex. It consists of over 200 different substances, including protein, fat, carbohydrates, vitamins, minerals, other nutrients, enzymes and hormones. This composition is not constant, but rather differs from mother to mother, and even varies within the same mother. It changes during each feeding, throughout the day, and over time to meet the needs of a growing child.

What are the stages of breast milk?

The production of breast milk begins in pregnancy and is defined in three stages: colostrum, transitional milk and mature milk.

Colostrum: Colostrum is referred to as the first milk. It's present at the end of pregnancy and during the first few days after the birth of your baby. It's usually thick, yellow and sticky, but it can also be thin and white or orange in color. Colostrum is easily digested by newborns. It's high in protein, low in fat and contains a high concentration of antibodies, specifically Immunoglobulin A (IgA), as well as white blood cells, to fight off infections. It's also a natural laxative which helps prevent jaundice by clearing the baby's body of meconium: the first thick, black, tarry stool. The volume of colostrum is small, but that small volume contains everything a new baby needs in the first few days of life.

Transitional Milk: Transitional milk is a combination of colostrum and mature milk. When your milk begins to “come in” at approximately 3 to 5 days after delivery, it mixes with the colostrum and gradually transitions to mature milk over the course of a few days or a week.

Mature Milk: Your milk will change over to mature milk by the time your baby is about 2 weeks old. Mature milk is a combination of foremilk and hindmilk. When the baby latches on to nurse, the first milk to flow out of your breast is foremilk, which is thin, watery and lower in fat and calories. As you continue to breastfeed, the hindmilk will follow. Hindmilk is thicker, creamier and higher in fat and calories.

What color should breast milk be?

The color of breast milk can vary throughout the day, or from one day to the next. It's usually white, yellow or bluish—but depending on what you eat, it could have a green, orange, brown or pink tint. Occasionally, blood from rusty pipes or cracked nipples can appear in your milk. It may be scary, but it isn't dangerous. As long as your baby is not refusing the breast, it's safe to continue to breastfeed if your milk changes color.


American Academy of Pediatrics. New Mother’s Guide To Breastfeeding. Bantam Books. New York. 2011.

American Academy of Pediatrics. Your Baby's First Year Third Edition. Bantam Books. New York. 2010.

Lawrence, Ruth A., MD, Lawrence, Robert M., MD. Breastfeeding A Guide For The Medical Profession Sixth Edition. Mosby. Philadelphia. 2005.

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