I'm considering breaking up with my boyfriend over something hypothetical and unlikely to happen.
What's the hypothetical reason?
In case we had a child and they would be born handicapped and in pain.
Everyone thinks about worst case scenarios or potentially bad things that might happen to them, but that's not a reason to break up with someone. Breaking up with your boyfriend because you're worried about something that hasn't even happened will just confuse him and hurt both of you. You will most likely regret your choice and wish you hadn't done it.
It's just a thought experiment. It doesn't matter.
Well it sounded like you were seriously considering it based on your initial message. If you really are considering doing it, you should rethink it. Breaking up with someone for something that hasn't even happened will just leave you with regret.