Premature breast development in female toddlers and young girls may arise from any of several factors, including:
• Benign premature thelarche, a noncancerous condition caused by slightly elevated levels of hormones
• Serious hormonal conditions in which very excessive levels of hormones are produced
Young female children with benign premature thelarche may experience the growth of breast tissue in one or both breasts for several months.  Then the extra breast tissue either:
• Disappears spontaneously
• Stays at the size of a very small breast until it begins to develop into a breast of a normal size at the time of puberty
In contrast, rapid growth of a breast or breasts in a young female child may be due to a serious hormonal condition.
If you notice that the breast or breasts of your young daughter or young grand-daughter is/are developing prematurely, contact a pediatrician for a proper diagnosis. It is important for a doctor to determine whether premature breast development in a young female child is due to benign premature thelarche or a serious hormonal condition.
1. C. McCarthy. Babies with breasts. Parenting. 07/06; 39.