The hardening of nipples is related to pregnancy, but factors such as physiological periods and nipple edema can also lead to hardening of nipples. In the early stages of pregnancy, there is an increase in the secretion of estrogen, and the stimulation of estrogen will lead to the development of breast ducts, which will not only gradually enlarge the breasts, but also make the nipples protrude and the areola color deepen, and small nodules appear. At this time, the nipples will become unusually sensitive and will become erect when touched by hand, so they will feel harder. However, in women of childbearing age and adolescence, the nipple hardens during the physiological period due to contraction of the smooth muscle of the milk ducts, which is intermittent and can loosen at any time. Nipple pain can also be considered as nipple edema, which is caused by nipple cracking, nipple biting during baby sucking, nipple cracking, and nipple fibrous connective tissue invasion, resulting in edema, spasm, and hardening of the nipple area, which can also be accompanied by local pain.