You'll need to provide information to your doctor about whether your niece started to grow breast tissue on both sides of her chest or only on one side. Then with the help of a GP or pediatrician, you can identify whether the tissue is glandular breast tissue or only fat tissue. If the growth is present only on one side, then it may be a local tumor. If both breasts are growing, then she may be experiencing a hormonal disorder. Hormonal analyses (FSH, LH, prolactin, estrogen, progesterone, DHEA, testosterone) can be performed to identify reasons for a possible hormonal disorder.