Ach, 6 years is a long long long-ass time to not have come to terms with your husband's poly nature. I'm sure it hurts him that you constantly sabotage his relationships with your possessiveness.
You don't seem able to wrap your mind around the idea that him being with others does not take away from his relationship with you. Ideally, his others should add to, augment, the love and fun you have with him.
Unless the other girl is truly a cowgirl and really does want to steal him away... that happens. But if you meet her and see she is on board with sharing him, respecting you, it shouldn't be an issue.
The main stickler is how he handles his NRE with new people. If he neglects you during that phase, doesnt make you feel special, dates them but not you, takes too many phonecalls and texts during your couple time, then it's on you to speak up about this, and on him to be more loving and sensitive to your needs for romance and fun too!
All that being said, now you're about to give birth, and I agree with the previous posters. Is you h cognizant of how much work he will need to put into the baby/toddler? Many poly people take a break for a good 3 years or so from dating others after a new child comes along, unless there is a longterm established secondary who is really into helping with the baby/housework/cooking/shopping as well.
One of the cool things about newborns is the overwhelming NRE one gets for them. That NRE could supplant the NRE your h has for his gf for a while, I bet!