The age of breast development in girls is between 9 and 11 years old, so it is normal for breast development to occur at the age of 9. At this time, girls begin to reach puberty and their nipples increase in size, and small protrusions, called sebaceous glands, appear around the areola. The color of the areola will also deepen and is usually accompanied by rapid growth in height, weight gain, and growth of internal organs. In addition, the growth and fat accumulation of the pubic bone and lower skeleton will increase the hip circumference. 9-year-old girls’ breasts will gradually become larger as they begin to develop, so parents need to help their children choose the right underwear to promote good breast development. Parents are also required to carry out correct science, so that girls can understand breast development as well as knowledge about sexual development, and correctly understand the development process of the body to avoid psychological stress. The 9-year-old girl is still in the active stage, so it is important to avoid external impact on the breast area and not to sleep on the side for a long time to avoid pressure on one side of the breast, which may lead to poor development. 9-year-old girl has severe breast pain, discharge, lumps, skin redness, etc. during the process of breast development. The girl should go to the hospital promptly if she has any abnormalities such as pain, lumps, skin redness and swelling. In addition, it should be noted that although 9 years old is already at the stage of sexual development, if a girl eats too much food or medicine containing estrogen before, it will lead to abnormal increase of estrogen in her body, resulting in abnormal development. Therefore, it is necessary to focus not only on supplementing nutrition in daily life, but also on supplementing nutrition in a scientific and safe manner. Although fast-food chicken, duck, fish and other foods contain more protein, the possibility of unscrupulous businesses feeding them with large amounts of hormone feed cannot be ruled out. If children eat such food for a long time, especially girls in the breast and sexual development stage, after eating will make the body estrogen content abnormally increased, which is not conducive to health, may lead to premature sexual maturity and other diseases.