38 weeks pregnant, and he's divulging all of this to you, right now? Saying 'I do' to a marriage proposal from another woman, hurtful, inappropriate and downright cruel to the woman who is about to have his baby.
Ultimatums, don't really work--word from the wise here on that.
One way to start... "I'm having a baby with you, and was under the impression that we'd committedly raise this child together. It really stings to know that you are still communicating with 4-5 of your previous lovers. I can't ask you to stop talking to them, that would be unfair on my part. What can you do, to assure me, that you are as committed to me, as I am to you?"
((that tosses the ball back in his court. And whatever answer he gives, hun, is the answer you decide on.))