I don't know if this will sound odd, or if this is actually a pregnancy 'thing' ... but in the past month I've been experiencing a painfully numb bum whenever I'm sat for any longer than 20 mins 
I'm just over 13 weeks, and I have put about half a stone on already (nausea grazing!) but I was pretty slim to start with, so I wouldn't have thought it was much to do with the weight bearing down on my behind.
MIL said it could be something to do with sensitive nerve endings?
Its more an observation (that can be extremely annoying - especially in the car or at work) rather than anything that I would bother my midwife with! ANyone else had anything similar?