The concept tripping me up linguistically today is how a man of the early 1800s might have expressed “biological child” when referring to making sure a baby born in wedlock was, well, his. The term “natural” would not be appropriate because it would imply illegitimate biological child. “Biological” is unusable because it did not enter English in a provable way until 1819 (given that my story is set no more than 10 years previous, and the term had been coined in German and moved to French by then, it might have been used in spoken English amongst educated persons and just not written down in a record that survived). I don’t want to use a phrase such as “child of his body” because in the context of the flow of the sentence I need a one-word adjective. Using “blood” doesn’t quite work.
In the end I settled for no adjective at all, and perhaps it’s an argument for letting the words just say what they have to say: “…make sure any babe born of the union was his–and therefore the marquess’ grandchild.”