How does the breast make milk?

Human milk—the ultimate smoothie—contains more than 200 ingredients. But how do those vitamins, minerals, proteins, fats, carbohydrates, antibodies, enzymes, and living cells get into the breast milk? Some of them are produced locally, right in the breast. Others are transported to the breast via the mother’s blood.

Human milk is manufactured in the alveoli—grapelike clusters of cells scattered throughout the breast. Only small amounts of milk are produced at first, starting around the fifth or sixth month of pregnancy. This early milk is called colostrum.

The placenta produces two hormones, estrogen and progesterone. These hormones keep milk production low during pregnancy by suppressing the release of prolactin, the hormone that causes milk production.

During pregnancy, the placenta transfers nutrients from mother to baby and removes waste products. After the baby is born, uterine contractions push out the placenta. Once the placenta is gone, estrogen and progesterone levels fall and prolactin levels rise.

The pituitary gland located at the base of the brain acts as the control center. When a baby breastfeeds, a message is sent to the brain. The brain receives the message and responds by releasing two hormones, prolactin and oxytocin. Prolactin stimulates milk production. Oxytocin causes milk release.

Estrogen and progesterone control breast milk production during pregnancy and after birth. But once a mother’s milk supply is well established (several weeks after birth), hormones play a minor role and milk removal (through breastfeeding or breast expression if a baby is unable to breastfeed) serves as the stimulus for milk production. The more milk a baby removes from the breasts through breastfeeding, the more milk a mother makes!

The first two weeks of breastfeeding set the stage for future milk production. Prolactin and oxytocin work hand-in-hand with the breastfeeding baby to ensure that mothers have an ample supply of milk.

Illustrations by Mandy Root-Thompson © baby gooroo. Content excerpted from Breastfeeding, A Parent’s Guide by Amy Spangler © 2010. Images and text may not be reproduced without permission. To purchase an e-version of this book, click here for Amazon Kindle, Barnes & Noble NOOK, or the iTunes store.

Last updated May 17, 2020

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