Dear Lou,
You must be bowled over. Don't forget the good, basic advice - I remember lists (Not his!) - given above, if you haven't done so already:
Make sure you've changed the locks to protect a) yourself, b) the cats, c) furniture/any other possessions or papers/overlooked foodstuffs.
Check all money/house situation. Put whatever stops on accounts you can, so that he doesn't fleece them, and perhaps you should transfer any money which is your own, to hold it somewhere safe. Inform the mortgage people. Arrange for copies of whatever house or other financial papers he has taken.
Tell everyone what he has done, including work. You don't want your personal or professional reputation damaged by this man and his deluded view of reality, which I suspect he will be very ready to share with others. Like Pride and Prejudice - if he is prepared to abuse you to your face, doubtless he will be very ready to do it with everyone else.
Follow up with finding a solicitor when you have the energy.
Take some time. I'm not sure where you are in this pregnancy but I think it's early? You have time to get yourself on to a more even keel before any decisions have to be made; you need to give yourself time to think and to decide what is best to do. If I were you I would tell the fewest number of people possible, and not tell Mr Homewrecker at least until much, much later, when all decisions have been made, if indeed you tell him at all. There is no need to tell people unless you want to, until you (or perhaps fate) have decided one way or another. You don't ever have to go into details: if you do decide not to go ahead with this, then just call it another mc to anyone who finds out. If you do decide to go ahead with it, then it is your decision taken after much consideration and I am sure you will be a wonderful mother. The decision and outcome is absolutely no one else's business but yours, in the circumstances.
FWIW we had years of ttc, and it caused great strain and sadness, and bitterness and arguments. I should think that happens to many couples. But neither of us went off with anyone else. And we weren't cruel and calculated and utterly egotistical in the way that you have experienced: that's another ballgame altogether.
Later on, steel yourself for the future. You can do it. I think you sell yourself short, because I and a stunning number of people on this thread can all see what an impressive person you are and how much you have to offer now and in the future. I remember right at the beginning when you said how you could hear your mother crying in the next room. That is what mothers do, bless her, and I'm glad that you have a great family and friends who have rallied round. Don't worry about being 'unkind' to your parents: you are not being so, and they will recognise it for what it is - a stage of your reactions to all this. But make sure that you let this cruel, damaged man (who I suspect cannot be mended) into your life and head as little as possible, and don't ever let him into a position where you are lying there listening to a child cry because of him. Be kind and generous to yourself: you deserve it.
Sorry for the essay.