What's Going On in There?
When you were pregnant, your placenta told your body to start producing estrogen and progesterone, which triggered changes. Among other things, some of the fat in your breasts was replaced by glandular tissue, and as a result your breasts probably became larger and heavier — perhaps several cup sizes larger and more than a pound heavier each — and may have felt tender. Your nipples and areola, the circular pink or brown area around the nipple, probably darkened and got larger. You probably also began to see bumps on your nipples. These are called Montgomery glands, and they often become noticeable during pregnancy. They produce a substance that keeps your nipples clean and soft.
How Milk Gets Made
The hormones produced during pregnancy also stimulate the growth of milk ducts, the paths breastmilk take from the “dairy” in the alveoli, your milk-producing cells, to the milk pools under your areolas.
When your baby begins nursing after you give birth, the nerves in your breasts signal your glands to begin producing the “true” milk, which starts filling your breasts sometime between the second and fifth day postpartum.
Until that milk comes in, your baby will be nourished with colostrum, a sticky yellow or orange liquid that is your baby's first milk. It contains antibodies that protect your baby against disease. Colostrum will also help your baby's body eliminate meconium, the black, tar-like substance that will make up his first poops. Your breasts only produce about two tablespoons of colostrum in the first twenty-four hours after you give birth, which is just the right amount for your baby.
Don't worry that you aren't producing enough milk in the first few days postpartum — your baby's stomach is very small at this point, and babies are designed to pack on fat during the last few weeks of your pregnancy in anticipation of the wait for your milk's arrival.