Making breast milk: When and how you start producing it

Your breasts have been preparing for nursing since you were in your mother's womb. By the time you were born, your main milk ducts – a network of canals designed to transport milk through your breasts – had already formed.

Your milk glands stayed quiet until puberty, when a flood of estrogen caused them to grow and swell. During pregnancy, those glands shift into high gear.

When do you start producing milk?

Your body starts making colostrum about three to four months into pregnancy, and your breasts may leak colostrum during the final weeks of pregnancy. (This happens to some women as early as the second trimester.) Your body will continue to produce colostrum for the first few days after birth. This high-protein, antibody-rich liquid contains all the nutrition your newborn needs.

Mature milk comes in approximately two to four days after your baby's birth, depending on the frequency of nursing in the first hours and days after birth. Your milk production will adjust to accommodate your baby's appetite and how often they nurse.

The process of making milk starts during pregnancy. If you're pregnant, you've likely noticed changes in your breasts, such as tenderness, swelling, and darkened nipples and areolas (the circle of skin surrounding your nipple). These changes may be some of the earliest clues that you've conceived.

No worries if you don't notice any changes in your breasts during pregnancy, though. That's also perfectly normal, and your body is still preparing to make milk.

Perhaps even more remarkable than any visible transformation are the extensive changes taking place inside your breasts. The developing placenta stimulates the release of the hormones estrogen and progesterone, which in turn stimulate the complex biological system that makes milk production possible.

Before pregnancy, supportive tissue, milk glands, and protective fat make up a large portion of your breasts. The amount of fatty tissue is an inherited trait and varies among women, which is why breasts come in such a variety of sizes and shapes. The size of your breasts doesn't determine your ability to produce milk or breastfeed.

What are breast milk ducts?

Nestled amid the fat cells and glandular tissue are the milk ducts, an intricate network of channels. Pregnancy hormones cause the milk ducts to grow in number and size.

The ducts branch off into smaller channels near the chest wall called ductules. At the end of each ductule is a cluster of small, grapelike sacs called alveoli. A cluster of alveoli is called a lobule; a cluster of lobules is called a lobe. Each breast contains up to 20 lobes, with one milk duct for every lobe.

You can think of the milk ducts as individual straws, some of which merge so that about eight or nine end at the tip of your nipple to deliver milk to your baby.

Your milk duct system becomes fully developed sometime during your second trimester, so you can make milk for your baby even if they arrive prematurely.

By the time your baby is born, your glandular tissue will likely have expanded significantly. Each breast may get as much as 1 1/2 pounds heavier!

What is colostrum?

Colostrum, sometimes called "liquid gold," is the first milk your body produces. It can be clear, white, yellow, or orange, and it's often thick and sticky. During the first days of breastfeeding, your newborn will be perfectly nourished by this nutrient-rich, high-protein, low-fat, easily digestible liquid.

This "first milk" is produced as the cells in the center of the alveoli dissolve and flow through the milk ducts to the nipple. Colostrum is higher in protein, minerals, salt, vitamin A, nitrogen, white blood cells, and certain antibodies, and it has less fat and sugar than mature milk.

It's also full of unique disease-fighting antibodies called immunoglobulins that strengthen your baby's immune system. When you feed your newborn colostrum, it's as though you're giving them their first vaccination.

Colostrum also has a slightly laxative effect that helps a newborn clear their gastrointestinal tract of meconium – the stool that accumulated in their bowel while your baby was in the womb. Getting rid of this black tarry poop soon after birth reduces the risk of jaundice.

Your baby's tummy is tiny, so they'll take just a tiny amount of milk at first – about a teaspoon of colostrum each time they nurse. But a little colostrum goes a long way. You may not feel as if you're producing much (and your breasts won't feel full yet), but each drop is packed with nutritional and protective components.

During the next few days, your milk will transition from colostrum to mature milk (though some colostrum will continue to show up in your milk for a few weeks).

How is breast milk made?

Milk ducts branch off into smaller channels called ductules. At the end of each ductule is a cluster of small, grapelike sacs called alveoli. Prompted by the hormone prolactin, the alveoli take proteins, sugars, and fat from your blood supply and make breast milk. A network of cells surrounding the alveoli squeeze the glands and push the milk out into the ductules.

As your body moves through the stages of lactation, it pumps extra blood into the alveoli, making your breasts firm and full. Swollen blood vessels and extra swelling in the breast tissue itself, combined with an abundance of milk and IV fluid given during labor, may make your breasts temporarily painful and engorged. But your baby will be nursing frequently in the first weeks, which will help relieve the discomfort.

What are signs my milk is coming in?

Within two to four days of delivering your baby, your milk will "come in." This means that your body transitions from producing colostrum to producing the mature breast milk that will nourish your baby going forward. (Second-time moms may find that their milk surge comes earlier than it did the first time around.)

Signs your milk is coming in include:

• Your breasts become firmer and fuller.
• Your milk production increases.
• Your breast milk becomes whiter and creamier.

What is let down?

For your milk to flow from your breasts, it must be "let down" or released from the internal alveoli.

Here's how it happens: Your baby's sucking stimulates the pituitary gland to release oxytocin and prolactin into your bloodstream. (The same thing can happen when you merely think of your baby nursing or hear their cries.) Oxytocin causes the cells around the milk-filled alveoli to contract and squeeze. The nourishing breast milk then flows into the ducts.

When your baby suckles, they compress the nipple and areola, creating negative pressure. That – along with the internal rush of milk from the letdown reflex – delivers the milk into your baby's mouth.

As your milk flow increases, the contraction of the milk-filled alveoli may create a tingling, stinging, burning, or prickling sensation in your breasts. Your milk may leak or even spray during let down. (If this happens at an inconvenient time, try crossing your arms in front of your breasts and applying gentle pressure to stop the flow. You can also put nursing pads inside your bra to absorb the milk.)

You may feel calm, satisfied, and joyful as you nurse due to oxytocin. No wonder some people call oxytocin the hormone of love! You also may feel sleepy and thirsty.

You may experience some cramping in your uterus as your newborn breastfeeds in the days after birth. This means that oxytocin is helping to shrink your uterus back to its pre-pregnancy size. (This same hormone caused your uterus to contract during labor.)

The discomfort is usually mild, but it can be painful, especially if this isn't your first baby. Your provider may recommend ibuprofen or prescribe a pain medication.

What's in breast milk?

Mature breast milk is perfectly designed and contains water, fat, carbohydrates, protein, vitamins and minerals, and amino acids. It also contains white blood cells, antibodies, enzymes, and other substances that boost your baby's immune system.

There are more than 200 known beneficial components in breast milk, with more being discovered all the time. For example, researchers now believe that a fatty acid in breast milk promotes the growth of a baby's brain and retina, and may even enhance cognitive development. Many of these elements, including infection-fighting white blood cells, can't be manufactured. (Learn more about the benefits of breastfeeding for moms and babies.)

During the course of a feeding, breast milk changes from foremilk, high in water and lactose, to hindmilk, high in fat and calories. The quantity of milk you produce increases along with your baby's weight and appetite until solid food becomes a daily part of their diet.

Breast milk is the ultimate personalized food. One of the wonderful characteristics of human milk is the way it changes to meet your baby's needs as they grow.

The breast milk a mother produces for a premature baby differs from the milk she would produce for a full-term newborn, and that differs from the milk she'll have for her 6-month-old baby. All breast milk, however, contains exactly the nutritional and protective components needed most by each baby at every age.

Breast milk even changes over the course of the day, with "wakeful" amino acids peaking during the day and ebbing at night. Conversely, tryptophan, a precursor to sleep-inducing melatonin, peaks during the night. If you pump your breast milk for your baby, you may want to label it with the time of day it was pumped and give it to your baby at roughly the same time to maximize its benefits.