Breast development in 9-year-old girls may be a normal phenomenon, and generally will not have an impact on height. Under normal circumstances, some girls at the age of 9 may enter puberty, breasts will gradually begin to develop, breasts may slowly bulge, is a normal physiological phenomenon, and will not have an adverse effect on the development of height. In puberty, the girl should also strengthen the daily care, keep the chest clean and hygienic, do not be overly compressed, often physical exercise, reasonable diet, supplement adequate nutrition, avoid the use of breast corsetry, less consumption of high-sugar, high-fat food. During puberty, when girls have menstruation, they should pay attention to physiological hygiene, change underwear and sanitary napkins to avoid infection. Parents need to pay attention to the growth of the child’s height, if the child’s height is significantly lower than the same age, need to pay attention to timely consultation, to avoid affecting the normal growth and development of the child.