The absence of nipples in a 20-year-old girl is considered a symptom of nipple entropion. The nipple is not visible or the nipple is invaginated because the muscles around the nipple and areola are poorly developed, causing the nipple to be pulled inward. Long-term nipple invagination is prone to local inflammation and inflammation in the ducts of the breast, and over time, nipple invagination can affect breastfeeding in the future. You should try to go to the regular hospital to guide the treatment, generally can be manipulated pulling or negative pressure suction treatment, can make the nipple protrude, breast ducts and fiber muscle lengthening, so that the nipple natural convexity to achieve the purpose of treatment. If the above methods still do not solve the problem, surgery is generally used to correct nipple invagination. Regardless of which method is used, it is recommended to visit a regular hospital first, so that the doctor can examine and then guide the corrective treatment.