Concave nipples are considered to be caused by the following reasons: First, it is considered to be a congenital factor, which refers to the fact that the nipples are concave after birth due to congenital developmental abnormalities in the embryonic stage, such as shortening of the mammary ducts or shrinkage of some of the nipple tissues, or underdevelopment of the nipple’s smooth muscle. Secondly, the nipple may be concave due to the inability of the tissue at the root of the nipple to pull and stretch the nipple during the developmental stage. Third, it may be due to acquired breast lesions, which are caused by nipple trauma, nipple inflammation, not treated in time, wearing inappropriate underwear or too tight pressure to the normal development of the nipple, or it may be triggered by mastitis or breast tumor. It is recommended to go to the hospital for further examination and take corresponding measures to pull out the nipple.