The federal definition of child pornography and what constituted sexually explicit depictions came up as part of a previous question of mine. Judging from that answer it appears that the definition of child pornography does not forbid depictions of breasts. Sexually explicit conduct is defined in 18 USC 2256, and section A include "lascivious exhibition of the genitals or pubic area", but does not reference breasts. Section B does include "lascivious simulated sexual intercourse where the genitals, breast, or pubic area of any person is exhibited", but this only applies for simulated sexual intercourse.
This implies that it would be fully legal to take and distribute images of a fourteen or fifteen year old teenager in sexually explicit poses with breasts fully depicted, in what I would intuitively consider to count as child pornography, and frankly I would want something like that to be illegal!
Is it actually true that such depictions are legal? I imagine if such depictions were legal some studio would have tried doing it, it would lead to general outrage, and the laws would soon be modified to prevent it. The fact that I haven't heard about such public outcry leads me to suspect that this sort of thing is not being done. If I had to guess such depictions of underage children is still considered illegal under different laws, perhaps constituting child abuse?
I'm asking on the federal level. I would not be surprised if some, or perhaps many, of the states have laws which would ban such underage depictions, but unless every state had such law (which I don't believe to be the case since the linked question explicitly stated that Georgia did not) there would still be potential for such depictions to be made and distributed in certain states.
Is there any federal law, or precedent, that would prevent sexualized depictions of underage teenagers with breasts fully exposed being produced?