By Willow Older

The process starts during pregnancy
If you're pregnant, you've probably noticed a metamorphosis in your bra cups. Those physical changes — tender, swollen breasts, and darkened nipples andareolas (the circle of skin surrounding your nipple) — may be one of your earliest clues that you've conceived. Experts believe the color change of the areola may also be a helpful aid to breastfeeding. It's nature's way of providing a visual guide that helps newborns nurse successfully (hey, dinner's over here!). Another pregnancy signal: the appearance of tiny bumps around your areola called the glands of Montgomery (named after the British obstetrician who first described them), which also play a role in breastfeeding. These bumps produce an oily substance that cleanses, lubricates, and protects the nipple from infection during nursing.

What's going on inside your breasts
Perhaps even more remarkable than this visible transformation is the extensive changes taking place inside your breasts. Your developing placenta stimulates the release of estrogen and progesterone, which in turn stimulate the complex biological system that makes lactation possible.

Before pregnancy, a combination of supportive tissue, milk glands, and protective fat makes 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.) In fact, 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 that transport milk through your breasts — had already formed. Your mammary glands stayed quiet until puberty, when a flood of the female hormone estrogen caused them to grow and swell. During pregnancy, those glands shift into high gear.

By the time your baby is born, glandular tissue has replaced most of the fat cells and accounts for your bigger-than-ever breasts. Each one may get as much as 1½ pounds heavier!

Nestled amid the fat cells and glandular tissue is an intricate network of channels or canals called milk ducts. Pregnancy hormones cause the milk ducts to increase in number and size; the ducts then branch off into smaller canals near the chest wall called ductules. At the end of each one 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.

Milk is produced inside the alveoli, which are surrounded by tiny muscles that squeeze the glands and push milk out into the ductules. Those ductules lead to a bigger duct that widens into a milk pool or milk sinus directly beneath the areola. Milk pools act as reservoirs that hold milk until your baby suckles it through tiny openings in your nipple. (You can think of the 15 or 20 milk ducts as individual straws that all end at the tip of your nipple and deliver milk into your baby's mouth.) Mother Nature is so smart that your milk duct system becomes fully developed sometime during your second trimester, so you can nurse your baby even if he arrives prematurely.

Production heats up after the baby is born

Milk production and prolactin
You'll begin full-scale milk production within 72 hours of delivering your baby. Scientifically speaking, this period is called lactogenesis. Once you expel your hormone-producing placenta, the estrogen and progesterone levels in your body suddenly drop. At the same time, the level of the hormone prolactin rises. This pituitary gland hormone signals your body to make lots of milk to nourish your baby. Laboratory studies also show that prolactin may 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, combined with an abundance of milk, may make your breasts temporarily painful and engorged, 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 (some women have this happen 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 and out the nipple. The precious, easily digestible liquid is chock-full of disease-fighting antibodies called immunoglobulins that strengthen your baby's immune system. (Click here to 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. When it reaches your breast, oxytocin causes the tiny muscles around the milk-filled alveoli to contract and squeeze. The nourishing liquid is emptied into the ducts, which transport it to the milk pools just below the areola. When he suckles, your nursing infant presses the milk from the pools into his mouth.

During the first days of nursing, you may feel some cramps in your abdomen as your baby sucks. This usually mild discomfort signals the release of oxytocin, which helps shrink your uterus back to its pre-pregnancy size. (This same hormone caused your uterus to contract during labor.) Another signal: You may feel calm, satisfied, and joyful as you nurse. No wonder some people call oxytocin the hormone of love!

As your milk flow increases, you may also feel some tingling, stinging, burning, or prickling in your breasts. Your milk may drip or even spray during letdown. Many women compare breastfeeding to learning how to ride a bike: It may be tricky at first, but once you — and your baby — get the hang of it, it becomes second nature.