Do they have kids of their own? How understanding do you think they would be if you pulled out last minute?
The other thing is from memory our reception place only needed final numbers a couple of days before, so you could do a tentative yes and then let them know a week before depending on how you are feeling... Could be good as something adult to look forward to post bubs (we had a wedding 8 weeks post and it was great
FWIW we had nanna babysit, but DD was settling/sleeping well in the evenings then so it worked)
You could also maybe take 2 cars (or DH taxi home) if you need to leave early?
I don't think people would really mind a 5 week old at a wedding... I think it's when they're old enough to be running around and/or you have to pay for them that people can get funny