From the very beginning of your pregnancy journey, your breasts are at work. They are evolving and preparing for the arrival of your baby. What changes can I expect? Your breasts go through a number of changes during pregnancy. These changes include

• Growth and enlargement
• Tender and hypersensitivity
• Darkening of nipples and areolas (the skin around your nipples), due to hormones that affect pigmentation of the skin
• Darkened veins along your breasts (due to increased blood supply to your breasts)
• Your breasts may start leaking a yellowish, thick substance known as colostrum
• Nipples stick out more; the areolas and nipples will grow larger
• Small glands on the surface of the areolas called Montgomery’s tubercles become raised bumps.
• Your hormones are the cause of these changes and you may experience some effects more than others.

Link