I had two cousins decline my wedding invitation when they were in late pregnancy and I completely understood even though I had never been pregnant. Now that I'm 33 weeks pregnant I totally understand why they couldnt make it. You need to look after yourself and bubs. I went out to a work function for my DH last weekend (and I was home by 11pm so it wasnt tooooo late) and it took me 3 days to recover so your body does react differently when pregnant. I agree with Aussienic and if you have already RSVP'd, just go to the ceremony and see how you feel. There is nothing wrong with leaving the reception at 8pm after making a token appearance.