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This is nothing to worry about in a girl her age as the first sign of puberty in girls is breast bud development. These occur behind the nipple and are painful on one or both sides off and on. So the outcome you are preparing for is puberty I'm afraid but don't worry about it at this point as the timeline for all of these events in a girl are breast buds and a little growth spurt about 2 years before pubic hair and then another year or so before the onset of her first menstrual period. That will make her 10 at least so perhaps a bit young but daughters generally follow the same pattern their mothers did.
Breast cancer and other serious problems are absolutely NOT a concern at her age and I assure you her doctor will (hopefully) tell you the same thing. No tests are really required unless you just need to know if she is really into puberty and in that case all usually do is get what is called a bone age film to look at the bones in both hands and wrists, on the same xray plate. This will show if her growth is advanced beyond her age and can help predict if this is going to progress more quickly or not. The issue with any testing is that you really are not going to do anything about it to slow it down, much as parents wish they could.
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