Many women believe that morning sickness (toxicosis) is the first sign of pregnancy. In reality, the breast often "senses" changes much earlier. Frequently, when a patient asks me, "Why do I have unusual breast pain?" , the answer lies not in pathology, but in hormonal transformation. 1. Why Does the Pain Start? As soon as pregnancy begins, estrogen and progesterone levels in the body spike sharply. These hormones are preparing your body for future lactation (breastfeeding).