Normal breast development occurs at the age of 9-11 years old, and the beginning of breast development in girls at the age of 9 years old is a normal situation judged by age alone, and is not precocious puberty in children. Usually, when the breasts first develop, the nucleus is palpable within the areola, and there is a slight sensation of tenderness, after which the nipple and areola begin to color, that is, the color deepens, and the nipple and areola also increase in size. This process is normal if there is a short period of time when the breasts are not equally large bilaterally. Some girls start to enter puberty at the age of 9, and due to the influence of hormones, in addition to breast development, pubic hair, axillary hair, and internal and external genital development may appear afterwards, and finally menstruation will occur, and their height will grow rapidly during this process. If a girl does not develop according to the normal development sequence, or if her height is abnormal, or if her nipples and areolas are not colored or enlarged when her breasts develop, you can take her to the endocrinology department of a hospital for a definite diagnosis of abnormal development in combination with bone age examination, pituitary MRI, adrenal ultrasound, uterine adnexal ultrasound, and sex hormone six tests. In addition, if there is bilateral unequal breast size and no improvement after a period of observation, it is necessary to pay attention to whether there are breast masses, such as fibroids, etc., which should be promptly treated by a doctor. 9 years old during breast development, it is recommended to wear soft and comfortable underwear, and parents should pay attention to sex education and psychological counseling for girls to prevent girls from developing low self-esteem due to breast development, face their own development situation, and correctly guide girls Parents should pay attention to sex education and psychological counseling to prevent girls from having low self-esteem due to breast development, face their own developmental situation, and guide girls correctly through puberty.