In the vast majority of cases, a lump in one breast in an 8-year-old girl is caused by premature sexual development, as most girls begin to show secondary sexual characteristics around the age of 10. The presence of a lump in a child’s breast at 8 years of age is often a sign of early development. At this point, you can check the blood levels of sex hormones, bone age and ultrasound of the breasts, uterus and ovaries to determine whether the child’s sexual development is consistent with his or her actual age. If the bone age is more than 2 weeks ahead of the actual age of the child, the child’s sexual development is too early, and timely intervention is needed, such as the application of Zhi Bai Di Huang Wan or Dafirin.