Hard nipples may be due to impending menstruation, or it may be related to pregnancy, breast diseases and other factors. 1. Impending menstruation: Under normal circumstances, nipples are relatively soft. When menstruation comes, the body’s estrogen level will rise, and estrogen will stimulate breast tissues, which may cause nipples to become hard. 2. Pregnancy: If a woman is pregnant, the level of estrogen and progesterone in her body will also increase, which will promote breast development and cause the nipple to be relatively sensitive, and when you touch the nipple with your hand, you may feel that the nipple is hard. 3. Breast disease: If you suffer from mastitis or breast hyperplasia, the onset of mastitis or breast hyperplasia will cause significant discomfort in the breast, such as nipple lumps, nipple development, breast pain, etc. The nipple hardness can also be caused by the presence of estrogen and progesterone levels. Hard nipples may also have other causes, it is recommended to consult a doctor in a timely manner, improve the breast ultrasound, blood tests or mammograms and other related examinations, under the guidance of a professional doctor to clarify the cause of the disease, targeted treatment.