The swelling of breasts after pregnancy is due to the synthesis and secretion of large amounts of estrogen and progesterone by the placenta, which acts on the mammary glands. When the last menstrual period is taken as the beginning of pregnancy, the fertilized egg is laid on the endometrium after 3 weeks of pregnancy and the placenta is formed after about 10 weeks and begins to secrete large amounts of estrogen and progesterone. Before that, the hormone level in the mother’s body increases, but it is not enough to cause significant enlargement of the breast cells and ducts, and the pregnant woman’s breasts will not be significantly swollen when she first detects her pregnancy. Estrogen is produced by the corpus luteum of the ovary in the early stages of pregnancy in small amounts, and is synthesized in large amounts by the fetal-placental unit after 10 weeks of gestation; progesterone is produced by the corpus luteum of the ovary in the early stages of pregnancy in small amounts, and is synthesized and released in large amounts by the trophoblast cells of the placental syncytium after 8-10 weeks of gestation. The nipples are enlarged and easily erected, and the areolas become darker and have nodular protrusions on the surface. During pregnancy, the placenta secretes a large amount of estrogen to stimulate the development of mammary gland ducts and a large amount of progesterone to stimulate the development of mammary gland alveoli. In addition, pituitary prolactin, human placental lactogen, as well as insulin and cortisol are involved in the development of the mammary glands. Some pregnant women experience lactation in late pregnancy under the effect of prolactin, but most of them do not have this symptom. It is important to take care of your breasts during pregnancy and wash your nipples daily.