Ever wonder how the mother’s body knows when to produce milk for her coming baby? Learn about the hormones and cells involved in milk production and release in this lesson on lactation and nursing.

Mammary Glands

So, your brand new baby has just been born! How exciting! Now what? Well, one of the first things a new mother has to do is learn to breast feed.

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Just as the baby has to prepare to enter the outside world and adapt to its new environment, the mother’s body has to prepare to take care of her newborn. Around the sixth month of pregnancy, the mother’s body prepares to produce milk for the coming newborn.

First, the mammary glands in the mother’s breasts enlarge and have fully developed by month six. These glands are like enlarged and slightly modified sweat glands. They contain cells that produce colostrum and milk under the influence of the hormone prolactin. Because these glands secrete their product, that’s milk, outside the body, they are called exocrine glands.

Okay, I know that’s a lot of keywords in one definition, but don’t worry, we’ll come back to them. But first, let’s continue with how the mammary glands work.

Myoepithelial cells in mammary glands aid in milk let-down.
Myoepithelial Cells Diagram

Once milk has been produced, it needs to travel from the mammary glands to the nipple, right? Well, this is under the influence of another hormone, called oxytocin, as well as the presence of myoepithelial cells, which surround the mammary glands. Myoepithelium are similar to muscle cells. They contract and push the milk out of the mammary glands and down ducts towards the nipple. This process is known as milk let-down.

Prolactin and Oxytocin

Okay, now that we know what happens, let’s get back to some of those keywords. First up, the hormones! Ahhhh hormones, they seem to be involved in everything, don’t they? In this case, we have two different hormones, both of which originate from the pituitary gland in the brain and travel to the mammary glands through the bloodstream.

Diagram of the anterior pituitary gland
Anterior Pituitary Diagram

Prolactin comes from the front part of the pituitary, which is called the anterior pituitary. Prolactin (PRL) is a protein hormone whose major target cell is the mammary glands. Secretion of prolactin increases during pregnancy and nursing. It stimulates mammary gland development and enlargement, as well as milk production.

Diagram of the posterior pituitary gland
Posterior Pituitary Diagram

Now, just being able to produce milk isn’t enough. The mother’s body has to get that milk from the mammary glands to the nursing baby. That is where our other hormone, oxytocin, comes in. Oxytocin is released from the back part of the pituitary, which is called the posterior pituitary.

Secretion of oxytocin (OT) increases towards the end of pregnancy when it stimulates uterine contractions during labor, and after pregnancy when it stimulates milk let-down from the mammary glands to the nipple.

So, your next question may be how does the mother know when to release milk? This is where our newborn baby comes in. When the baby cries, the mother’s brain recognizes the sound and it triggers the release of both prolactin and oxytocin. These hormones work together to trigger lactation, or the production and secretion of milk from the mammary glands. Let’s look at the steps involved:

  1. Crying of the baby, as well as thoughts of your baby or even the crying of another baby, can stimulate the hypothalamus, which in turn stimulates the release of prolactin and oxytocin from the pituitary.
  2. When the baby starts to suckle at the nipple, a physical stimulus is sent to the mother’s brain, which causes more oxytocin and prolactin to be released.
  3. Prolactin and oxytocin travel to the mammary glands, where prolactin stimulates the production of milk in the glands and oxytocin stimulates the myoepithelial cells to push milk from the glands, down the ducts and toward the nipples.
  4. Milk gathers at the nipple and is released into the baby’s mouth. And then, hopefully the crying has stopped. At least for now anyway!
Milk production and secretion occurs in a positive feedback loop.
Nursing Positive Feedback Loop

This is called a positive feedback loop because as long as the baby keeps suckling, milk release will continue. To stop the positive feedback loop, the physical stimulus of suckling needs to be removed.

Colostrum and Breast Milk

Okay, so now that you know all about how a mother produces breast milk, let’s look at breast milk itself. Remember back to that definition of the mammary glands? That one with all the keywords in it? See that one there? Colostrum.

Well, did you know that there are different kinds of breast milk? And that colostrum is one of them? Colostrum is a secretion that is thicker and contains higher levels of protein and less fat than normal breast milk. It is produced during the first few days after birth and contains high levels of antibodies to help newborns ward off infections until their immune system becomes fully functional.

It also helps prepare the newborn’s digestive tract by sealing the permeable spaces in the intestines and initiates the removal of the waste product meconium, or the first stools, from the intestinal tract.

Near the end of the first week after birth, colostrum production decreases and normal breast milk production and secretion increases. While colostrum consists mostly of proteins, including antibodies, breast milk has a lot more ingredients.

Protein levels are lower and water and fat levels are higher in breast milk.

It also contains:

  • Sugars
  • Amino acids
  • Salts
  • And high levels of lysozyme, an enzyme that has antibiotic properties


So, now that we know how breast milk is produced and how the milk let-down reflex works, what about how the mother knows when to stop producing milk? This process of removing the mother’s milk from the baby is called weaning and normally takes place one to two years after birth.

However, the timing of weaning is often left up to the mother, and mothers can choose to wean early, a common practice in countries where baby formula is available, or later. In some cases, as long as the baby continues to suckle, the mother will continue producing milk.

However, some mother’s bodies will stop producing milk all on their own despite the suckling stimulus. This can be due to the presence of other hormones in the body that inhibit prolactin.

When a mother chooses to wean her baby early, while it may be convenient for the mother, it is important to note that infant formulas lack the antibodies that breast milk contains, and this can be less healthy for the baby.

Lesson Summary

And there you have it! Now you know how, why and when breast milk is produced. The mammary glands complete development during the last six months of pregnancy. After pregnancy, they produce and secrete breast milk for the newborn.

The production of breast milk is under the influence of the hormone prolactin, which is secreted by the pituitary, causes the maturation of the mammary glands and the production of breast milk. The release of breast milk, however, is under the control of another hormone, oxytocin. Oxytocin initiates the milk let-down reflex in response to the suckling of the baby.

Milk from the mammary glands is pushed out of the glands and down the ducts towards the nipple by the contraction of cells called myoepithelial cells.

During the first few days following the birth of the baby, the milk secreted from the nipples is thicker than normal breast milk. This is called colostrum, and it contains high levels of proteins and antibodies to help the developing infant’s immature immune system fight infection.

At the end of the nursing period, mothers will wean their babies. Weaning is the removal of the mother’s breast milk as the baby transitions to either infant formula or solid foods. But remember, infant formula doesn’t contain the antibodies that breast milk does, so it’s important to breast feed your babies for at least the first few months of their new life.

Learning Outcomes

Following this video, you will have the ability to:

  • Describe when the mammary glands fully develop and their functions
  • Understand the function of the myoepithelial cells
  • Explain the importance of prolactin and oxytocin in pregnancy and in the development of breast milk
  • Name the structures that release prolactin and oxytocin
  • Summarize the milk let-down reflex
  • Distinguish between colostrum and regular breast milk
  • Define weaning and know when it occurs
  • Explain why formula is not a perfect substitute for breast milk