Answer: pregnancy brings about a lot of different changes in the body. Some changes are expected, such as weight gain, change in size of breasts. The one good thing about sore breasts in pregnancy is that it is often the first indication that you are pregnant.Your breasts are preparing to feed baby. There’s a fat layer that thickens inside of the breasts. The milk glands multiply. Blood flow increases. And even nipples can change size, shape, and thickness. All these changes start to make your breasts sore and bigger.