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The process starts during pregnancy
If you're pregnant, you've probably noticed big changes in your bra cups. Those physical transformations – tender, swollen breasts and darkened nipples and areolas (the circle of skin surrounding your nipple) – may be some of the earliest clues that you've conceived.
Experts used to believe the color change of the areola provided a visual aid for babies, but there's no evidence to support this. Longtime lactation consultant Jan Barger points out that if you watch babies going to the breast shortly after birth, they have their eyes tightly shut!
Another signal that your breasts are getting ready for nursing is when the bumps around your areola get bigger and more noticeable, often in the first trimester. These bumps are called glands of Montgomery or Montgomery's tubercles. The oil they secrete lubricates your nipples and helps prevent drying, cracking, and infections when you're breastfeeding.
What's going on inside your breasts
Perhaps even more remarkable than this 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.
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Before pregnancy, supportive tissue, milk glands, and protective fat make up a large portion of your breasts. The amount of fatty tissue varies among women, which is why breasts come in such a variety of sizes and shapes.
Your newly tender, swollen breasts have been preparing for your pregnancy since you were a 6-week-old embryo in your own 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.
By the time your baby is born, your glandular tissue will have expanded significantly, which accounts for your bigger-than-ever breasts. Each breast may get as much as 1 1/2 pounds heavier!
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 between 15 and 20 lobes, with one milk duct for every lobe.
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, which lead to a bigger duct. (You can think of the 15 to 20 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 he arrives prematurely.
Production heats up after your baby is born
You'll begin full-scale milk production within 48 to 96 hours of delivering your baby. Second-time moms will find their milk surge comes earlier than it did the first time around.
Once you expel your hormone-producing placenta, the estrogen and progesterone levels in your body suddenly drop. At the same time, the level of prolactin rises. This pituitary gland hormone signals your body to make lots of milk to nourish your baby. (Laboratory studies show that prolactin may also make you feel more "motherly," which is why some experts call it the mothering hormone.)
As your body readies itself for 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, may make your breasts temporarily painful andengorged, but nursing frequently in the first few days will help relieve any discomfort.
First comes colostrum
During the early days of breastfeeding, your baby will enjoy a creamy, high-protein, low-fat substance called colostrum. You may have leaked a few drops of this thick, yellowish substance during the final weeks of your pregnancy. (This happens to some women even earlier, during their second trimester.)
This "first milk" is produced as the cells in the center of the alveoli dissolve and flow through the milk ducts to the nipple. The easily digestible liquid is full of unique disease-fighting antibodies called immunoglobulins that strengthen your baby's immune system. (Learn more about the different stages and characteristics of breast milk.)
How milk flows from you to your baby
For your baby to enjoy your milk, it must be "let down" or released from the internal alveoli.
Here's how it happens: As your baby sucks your nipple, he stimulates the pituitary gland to release oxytocin (as well as prolactin) into your bloodstream. (The same thing can happen when you merely think of your baby nursing or hear his cries.) When the milk reaches your breast, oxytocin causes the cells around the milk-filled alveoli to contract and squeeze.
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The nourishing liquid is emptied into the ducts. When your baby suckles, the combination of his compression of the nipple and areola and the negative pressure he creates by sucking – along with the internal rush of milk from the "letdown" reflex – delivers the milk directly to him.
During the first days of nursing, you may feel some cramps in your uterus as your baby sucks. This usually mild discomfort 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.)
You may feel calm, satisfied, and joyful as you nurse. No wonder some people call oxytocin the hormone of love! You also may feel sleepy and thirsty. These are signs that your baby is stimulating the breast appropriately.
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 drip or even sprayduring letdown. (If this happens at an inconvenient time, try crossing your arms in front of your breasts, applying gentle pressure to stop the flow.)
Just as nature intended, the more often your baby breastfeeds, the more milk your body will produce. It's an amazing process, just like the pregnancy that created the baby in your arms.