There are a few things you touched on but I have not seen anyone else comment on yet:
First, the man may have a cause of action for battery against the people who restrained him. If you see two people fighting, and you jump in to save the "victim" (who turns out to be the aggressor), you may be held accountable for battery. Mistake is not a defense in this scenario.
Second, the fact that she was pregnant does not absolve her from criminal or civil liability. If anything, judges are very likely to look at that as a sign of extreme negligence and will throw the book even harder.
Third, you have a right to defend yourself regardless of the physical condition of your attacker. If we made it so that someone could not defend themselves against a pregnant attacker, why wouldn't we make it so that people with cancer could attack people with impunity. How about people with asthma? Warts? It sounds silly, but once you draw a line of immunity it gets hard to decide what is protected and what isn't.
I agree that it makes sense to just call the police and press charges, but the way you described the situation makes it seem like she would have continued to hit him unless he physically stopped her.