If your wife and girlfriend are somewhat close, I would suggest she take an active role in supporting your wife as well (and makes it clear she'll be available to take care of the child after birth so you two can still have dates) so that it feels more balanced to your wife. And your being there for her throughout the pregnancy should help her.
In any case, I would suggest telling your girlfriend about the pregnancy, as it will affect her as well, and I feel if it was me, I would feel left out if I wasn't told as early as possible.
Other than that, I think your wife will probably have the same kind of reactions as other pregnant women tend too. What I mean is, sure you're poly and therefore she can accuse you of neglecting her for your girlfriend, but if you weren't she could either accuse you of the same with your pals, or of cheating on her, etc.
At any rate, it's important to realise these concerns are probably very real for her, and to be reassuring, and at the same time not to take them too personally as a lot of it might be due to the pregnancy and not your relationship.