It may be normal for a 12-year-old girl to be undeveloped, and girls generally begin to develop between the ages of 9 and 12. The delayed development of girls is related to genetic factors, dietary habits and living habits. 1. Genetic factors may be related to her mother’s late menarche and her father’s late development. 2. Dietary habits such as picky eating, irregular eating, etc. cannot provide enough nutrients for the development of the body, which needs to eat more nutritious food, such as meat, vegetables and fruits. 3. Bad living habits such as staying up late and not loving sports can be improved by changing bad living habits, ensuring enough sleep and enough sports. Dietary and living habits can be changed to promote growth and development. It is not normal for girls to develop too early, i.e. earlier than 8 years old, or too late, later than 15 years old, and parents need to take their children to the hospital in time for examination, identification of the causes and treatment.