Lori,
If you have no qualms of your significant other meeting an 'old' fling, that he's obviously still 'smitten' by - while you're X months pregnant - then I say go for it. However, if you do go, I just want you to know YOU ARE PLAYING WITH FIRE.
Do the right thing, ask him to take his wife or say you can't make it.
I'd say if he's sincere about meeting up for drinks and is just 'reminiscing' he won't have a problem bringing the wife who's caring for his child and carrying his second one - afterall, she needs some time away from the house too.