During early pregnancy, due to the rapid increase in estrogen and progesterone levels, estrogen and progesterone will act on the interstitial glands of the breast, causing them to become engorged, edematous and hardened. Some people will feel fullness or even pain in the breast, and the pain in the breast will occasionally irritate the nipple. Because the nipple has a skin muscle called the dermatome below the areola, when the dermatome contracts, the nipple becomes erect and protrudes, and the painful stimulation may trigger an erection, or protrusion, of the nipple. Protruding nipples can be a sign of early pregnancy, but a more definitive sign would be menopause and elevated HCG in the urine or blood, which would be an indicator of pregnancy.