這隻能叫乳房過早發育 - Premature thelarche
N years ago, my master's thesis was about this.
Here is a reference for you:
http://www.ncbi.nlm.nih.gov/pubmed/3137242
You can also google to pull out more publications. For example:
https://www.google.com/?gws_rd=ssl#q=premature+thelarche+causes