The appearance of a breast nucleus on one side of a 9-year-old child may be the beginning of breast development, which is a normal physiological phenomenon and does not need to worry too much. When breasts develop, it may be that the opposite breast starts to develop only after a few months interval of unilateral breast development, or both sides develop symmetrically, forming a breast nucleus, which is a small hard lump to touch and painful to press. Usually girls start to develop after the age of 8, mainly including breast enlargement, growth of pubic and axillary hair, widening of the pelvis, menstruation, and other symptoms, at the age of 9-10 years old, the nucleus of the breast appeared, and breast development began, at the age of 12-13 years old, the mammary ducts and the subcutaneous fat are increasing, the areola and nipple become darker in color, and the breasts are gradually bulging and rich in elasticity. When breast nucleus development occurs, in addition to the normal development of puberty, it is also considered that it may be precocious puberty, which also occurs in breast development, it is recommended to go to the hospital for further examination, to clarify the cause of the disease, and targeted treatment. In your daily life, you should have a balanced diet, eat less snacks and hormone-containing foods, and supplement more protein to promote normal development.