The possible clinical causes of breast bulging on one side of a ten-year-old girl include chest trauma, breast occupying lesions, and differences in bilateral breast development. 1. Chest trauma: If the child has obvious pain when touching the breast, and at the same time has a history of recent trauma, this is usually considered to be a localized edema of the chest tissue caused by trauma, and can be relieved by appropriate rest. 2. Chest occupying lesions: this kind of situation is extremely rare in children, clinically related diseases may include hemangioma, lipoma, sebaceous cysts, fibroma, etc., which may cause one side of the girl’s breast to appear abnormal bulging, and ultrasonography is needed to make a clear diagnosis. 3. Differences in development of both breasts: some girls may have uneven development of both breasts during the stage of breast development, which is closely related to their own living habits, such as sleeping position, sports status, etc. If one breast is pressurized, it may be underdeveloped. If it is a disease factor, parents are advised to go to the child as soon as possible for examination to make a clear diagnosis, so as to carry out targeted treatment at an early stage.