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I see why he used that word. What is the positive adjective for a non-adopted child? Hmm... I looked up natural child and it means one born out of wedlock.

I think they usually refer to it as "biological child".

I think the biological adjective fits best. I just feel there is probably another one that has been lost to the way words come out of disuse over time. See words like "progeny" and "posterity"