Welcome to our forum.
I think you have to figure out exactly what you want/need, express it to your wife, find out if it's a dealbreaker, compromise if possible. Do you want/need your wife to join you in dating outside the marriage, or is it fine if she stays monogamous, even if you're polyamorous? What do you really want/need in terms of dating for yourself? How does this affect your wife? Can you compromise so that both of you are reasonably happy?
Perhaps your wife would be willing to join this site, and/or read some of the literature that you're reading. Add "Opening Up
" (the book) to your reading list if you haven't already done so.
GalaGirl made a good point that a pregnancy is a tough time for your wife to have to process all this emotional information. Can it wait? Can you take things at a slow pace, so as to give her time to adjust?
GalaGirl's post was of good quality, so read it through a couple of times. I hope Polyamory.com proves helpful for you. There's lots of good info here.
Glad to have you aboard,