A basic knowledge of breast anatomy will help you to understand milk production.
The insides of your breasts are divided into sections like the inside of a grapefruit. Each of these sections is called a lobe, and each breast has 15 to 25 lobes. Inside each lobe are 20 to 40 smaller sections called lobules. The lobules contain glands called alveoli that produce milk, and these glands cluster together like grapes around the milk ducts. The milk ducts join together like grape stems and connect to the milk sinuses, located under the areola.
Your Changing Breasts
You may have already noticed your breasts changing during your pregnancy. The placenta stimulates your body to produce hormones (estrogen and progesterone) that prepare your breasts for lactation. Recent animal research indicates that the placenta itself may actually produce those hormones. Your body responds in several ways.
First, your nipples and areolas darken. This helps them to stand out from the surrounding tissue and act as a visual bull's-eye for your baby. Dramatic experiments in Europe highlighted the importance of the areola's darkening.
After delivery, babies in this study were placed face-down on their mother's abdomen. Amazingly, those little newborns zeroed in on their mother's nipples, crawled up, and latched onto a breast without any direction or assistance. These infants won't know how to control their own bodies for months and yet they found their mothers’ nipples without any help. Unfortunately, this incredible inborn ability disappears soon after a baby's first hour of life.
Second, your breasts grow larger and sometimes tender. Throughout pregnancy, some women gain well over one and a half pounds in each breast as glandular tissue is being added to enable lactation. These milk-producing cells will replace a large portion of the fat cells in your breasts.
Studies indicate that women who have been pregnant enjoy a decreased risk of breast cancer compared to women who have never been pregnant. These findings imply that your breasts are just like the rest of your body in one important way: They are healthier when used the way nature intended.
Third, your milk ducts grow. These ducts are the paths milk takes from the dairy in the alveoli to the sinuses under your areolas. In effect, your breasts are doing road construction during pregnancy, increasing both the number of ducts and their size.
Finally, your Montgomery glands become noticeable as bumps on your areolas. These glands produce an oily substance that cleans and lubricates your nipples.
At some point in your second trimester, the work is done and your breasts are ready to nourish your baby. The high levels of progesterone in your body prevent lactation from occurring before birth, but you may notice fluid leaking from your breasts. This fluid is called colostrum, and it varies in appearance from thin and clear to thick and white. Colostrum is formed when the cells inside your new milk glands dissolve. Some women leak a little colostrum during pregnancy, and others don't—either case is perfectly normal.
Internal Organization: The clusters of alveoli produce milk and the ducts transport it to the milk sinuses, where your breast milk is stored until your baby's next feeding.
External Diagram: The breast is divided into three sections: the nipple, the areola, and the Areolar Margin