My girlfriend declined our wedding because she was heavily pregnant, I knew it was highly likely, I invited her as a courtesy - so it didn't bother me. It just gave us the chance to invite others.

I also had to decline a friends wedding as we were delivering a few days before, we didn't want anyone to know so we just said, we can't attend incase I go early.

If they know your pregnant I am sure they will understand. My sister went into labour at our brothers wedding, you could always take that chance!