The development of one breast in a 6-year-old girl may be caused by precocious puberty, or it may be caused by other diseases, such as mastitis, etc., and you need to go to the hospital in time to find out the cause of the disease. Because of individual differences, the development process of children of different ages and at different times is not exactly the same. In general, the normal development time for girls is around 13 years old, and precocious puberty refers to the appearance of secondary sexual characteristics before the age of eight. The main manifestations in girls are breast development and the onset of menstruation. Precocious puberty is often associated with tumors, neurological infections, and congenital developmental abnormalities. What looks like breast development on one side in girls may actually be a clinical manifestation of some diseases. For example, children with mastitis may have painful enlarged breasts with hard nodules and tenderness, which may look like a girl’s breast development. When the child is in discomfort, the cause of the disease should be identified and treated immediately.