missingcaffiene - thank you love, I know you are in a similar position and our rotas can be a bit brutal. I am part time, at least (I work a job share).
I came off on calls/nights in my last pregnancy around 20 weeks as I was really struggling. To be perfectly honest, it was a horrible experience. I asked for a risk assessment and some alteration of my hours and they asked me to see one HR person after another about it - this all went on for around a month while I still worked the full rota. I made my own appointment with occupational health who said I shouldn't be working those hours, and then I was told that risk assessment was invalid as I had self referred and I shouldn't have gone to them without asking. They made personal comments about how I didn't have a very big bump so should be able to manage, they told me other people worked on calls until much later, they asked me why exactly I couldn't work nights (this from HR people who have never worked a medical shift in their lives) - I got very upset and vulnerable. I finally got my GP to write me a note and they took me off on calls with lots of nasty comments. They constantly rang me up to ask me to cover the on calls they had taken me off -and got huffy with me when I said i couldn't.
This is a different hospital now but it was such a horrible experience (I was still really upset long after the birth, and my confidence in my professional ability really took a knock because of it all for a while) that I feel like I'd rather just battle through for as long as possible. But later, maybe 3rd trimester, I will get my GP to sign me off nights, I don't think they're good for you at that point. I just feel like hiding and not telling anyone and as I don't seem to be growing a particularly big bump this time either I guess I can for a little while yet.
This must make me sound like a very weak pathetic person 
What a depressing post - so sorry! And you with your lovely scan - huge congratulations love. I also got my due date moved back about a week last time -it felt like winning a prize!! baby was a week late and born on my original date but never mind eh