One of the first physical changes of pregnancy is a change in the way your breasts feel. They may feel tender, tingly or sore. Or they may feel fuller and heavier. As early as two weeks after conception, your breasts start to grow and change in preparation for producing milk. The primary cause of these changes is increased production of the hormones estrogen and progesterone. Changes in your breasts are often most dramatic when you're pregnant for the first time.
Early signs and symptoms of pregnancy - Mayo Clinic