Wait, what? I can't think of any female POW's being held on US soil in any significant numbers, and the male POW's with pregnant wives giving birth back in, say WW2 Germany wouldn't be classified as citizens (and the kid is being born in a totally different country), so what I'm trying to say is your question is confusing.

Could you clarify?

Yes, the 14th amendment. "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside." For the most part, if someone is born here, they are a citizen. There are some rare exceptions, such as for the diplomatic corps, but I know of no limitation on the child of a pow. That child would be subject to the jurisdiction of the US as far as I know.