Breast gland thickening of 21mm is usually described during ultrasound and other imaging examinations of the breast, and is mainly seen in physiological gland thickening (menstruation, pregnancy, etc.) and pathological gland thickening (breast hyperplasia, breast tumor, etc.), which requires further improvement of the examinations in order to clarify the cause of the disease. 1. Physiological gland thickening: it is related to the hormonal changes in the patient’s body, especially during menstruation and pregnancy, the levels of estrogen and progesterone in the body change, which leads to the thickening of the mammary gland up to 21mm, and it usually returns to normal after menstruation and pregnancy. 2. Pathological thickening of the glands: more common in the clinic are breast hyperplasia, which belongs to the pathological state and is usually benign, and may lead to the thickening of the breast gland to 21mm. it can also be seen in breast tumors, including benign and malignant tumors, which will lead to the thickening of the glands of the breast. The thickening of the breast gland to 21mm should also be considered for other reasons such as inflammation of the mammary gland, and it is recommended that the patient go to the hospital, complete the auxiliary examination, in order to clarify the cause of the disease, and then give the relevant treatment by a professional physician for the condition.