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I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you.
I can assure you that this is nothing to worry about or cancer.
Many babies, boys and girls both, are born with some breast tissue lumps under the nipples. This is due to the influence of the mother's hormones in pregnancy.
In older girls there is sometimes a slight surge in estrogen production by the ovaries and breast tissue is extremely sensitive to hormone stimulation. It is not due to foods, formula, hormones in milk, or anything external as long as you are not using any estrogen creams that she might be exposed to.
The lump may regress or stay the same but should not enlarge much more if at all. As long as there is no tenderness or redness around the nipple then you need not worry.
Yes, breast cancer is unheard of in children so you need not be concerned about that, which is why I mentioned this in my second comment. It is strictly a hormonal thing and not serious and actually quite common.
Just leave it alone then and it will likely regress some but not completely go away. I assure you I've seen this countless times and it is entirely normal.
Glad to be able to dispense some peace of mind! If you have further questions, even after you (hopefully!) accept/rate my answer, you may still return here later at NO additional charge.
You are welcome. I'm seeing patients but will still be online but chat times us out every 5 minutes until you exit and accept/rate my answer so let me know if you have further questions.