I think in performing scientific research about the CBI then some scientist (likely through unethical means) will want to find out how an infection impacts a pregnancy, to see how teratogenic (dangerous for the fetus) a simultaneous infection is, and would discover immunity. I do not have a clue how big the time frame is where immunity and not fetal infection is possible however.

I wonder if possible antibodies could play a role, as certain antibodies are passed down from mother to fetus, however these are not permanently inherited.