You will notice a big change in your breasts when the baby is born. Once the placenta leaves the body, hormones stimulate the alveoli to produce milk. The milk passes through the lactiferous ducts and is stored in the small reservoirs situated under the areola causing the breasts to swell. You may feel that your breasts are full or even a little tight. If you breastfeed often, these sensations tend to go away.
When the baby is positioned correctly, movements of the mouth squeeze the reservoirs located just under the areola. The milk flows through several small openings located on the tip of the nipple. The position of the baby's mouth on the breast is very important to assure that the baby is breastfeeding well.
The following animation illustrates the production of milk.