I'm sorry to hear your spotting Bexxie, make sure you post us a piccie next friday. I'm sure everythings fine [-o<
My dad had a hernia op last december and had to have a few weeks off because he couldn't walk properly (he's bin man) and when he came back they all had to sign a document (which typical dad, he never read) and it states that anyone taking excessive time off sick will be disciplined.
He was given a warning, even though he was off sick before this new part of the contract came into force
So I think they can give you warnings etc but I'm pretty sure they can't give you a final warning and sack you whilst your pregnant. Especially as Jo says you should have a risk assessment done which would show that you need a job sitting down. If they were to sack you that would be classed as them being unreasonable and against the HMRC rules for employers and pregnancy and you would have a claim without a doubt.
I hope everything goes ok, but try not to worry and relax for bubbas sake *hugs*
Cx