I used to have to do this with my DS. I would have to lie on his bed with him till he fell asleep then sneak off.....I didn't have the trouble of him realising and waking up though.

I remember then one night, I really needed to get up and go to the loo.....so I told him, I was going to the toilet and I'd be back....he stayed quiet....waiting.....then I went back in there and he was asleep. After that for a few days I told him that I was off to the toilet and left him to go to sleep......now we say our ritual words....."Night, Night....sweet dreams....see you in the mooooorrrrrning....I loooovvveee Youuuuuu" and I walk away, he puts himself to sleep and I don't see him till the morning (unless there is something wrong)

If my DS does get up, I always make sure I put him back to bed, cover him up tell him it's time for sleep and he's OK.

It took until 23 months for him to do that though. I think consistency is the key and just keep trying till he gets it. I know it must be hard too with such a big belly. Your DS probably knows something is up and is trying to get as much attention from you as he can before baby comes along.

Good luck with it....and good luck with Baby too