1. How early in pregnancy does breast swelling and pain begin? In the early stages of pregnancy, about 4-6 weeks after conception, many women will feel breast swelling and pain. This is because the hormone levels in the body have changed. During early pregnancy, levels of estrogen and progesterone in the body continue to rise, and these hormones stimulate the mammary glands to enlarge and prepare to produce breast milk. This enlargement may cause mild pain or discomfort. two,