Most likely I would try to dance around it or find a different way to convey my thoughts (like for any other thing I couldn't express clearly) or use `it` because in my first language (french) `il` is the default pronoun for a baby (`il a faim ton bébé ?`) while thinking something's off and ask the person to forgive my blunder.

I would certainly not used something like `is they a boy ?` though.