Well, you've heard it out the kazoo, and it smarted with me too....you judge the 'mistress' as being very unstable upon another post that suggests that this mom to be is aware of the instability of her situation and may be acting out....
Your husband is lucky to have you...not that no one makes mistakes. We all do, all the time. This is a hard one. Your husband is more out of bounds than even the woman....as he has so much more at stake. Having an affair, and using no birth control.
I am truly glad this is making you and husband closer, but be careful of doing so at the risk of labeling this woman out there. Thats making her the scapegoat and not making your husband accountable for at least the same behavior. Your already compassionate by loving your husband through this, try some compassion for the blamed scarlet lettered woman who's fault was sleeping with your HUSBAND. And yes, she's pregnant and if she was against abortion, she should be freaking out. Not to excuse her behavior.
Try to be level headed about this. Both her and your husband were at fault, and you will need to come to terms with that, and his lack of carefulness, in couples counseling. For both of your sakes, I hope you will both be honest, stop the blame game, and own up.