If the nipple of a woman's breast does not protrude from the plane of the areola, or even sinks under the skin, resulting in a local crater-like appearance, it is called a crater nipple, which is a common female breast deformity. It has been reported that the proportion of women with inverted nipples is as high as 40%. There are two types of inverted nipples: a true inverted nipple is when the nipple retracts or turns into the breast, and does not rise above the breast skin even when pulled; a false inverted nipple is when the nipple develops normally but sinks into the areola, the nipple and the skin are on the same plane, and cannot stand up. When pulled out like a normal person, it is called a false inverted nipple. True inverted nipple is common on both sides, and is primary or hereditary; unilateral inverted nipple is less common and is usually secondary to breast disease.