I moved my son to a bed on his second birthday. Like you, to accomodate a coming baby. It's good that you're asking these questions before you make the move, to get others views on how to not create a sleep problem.
I set one rule for the bed for both my children. One rule. Do not get out of bed unless your bedroom is on fire. There was no "i need a drink" or "i have to go pee" Those issues should be addressed before bedtime....
One thing that helped trememdously was to create a bedtime routine. My kids adored going to bed and I never had one problem. I did it by using the routine below:
1) Never under any circumstances use the phrase "If you don't behave, you're going to bed" This just sets up bedtime right away as being something to use as punishment instead of something we all need to do every day of our lives. Wouldn't you see the bed as a bad place to be if you got put there when you behaved bad?
2) Same bedtime every night. This is critical.
3) I had a clock set in the livingroom for my son and it would go off twice. Once, 1/2 hour before bed, which was "get ready" time. Then it went off again at bedtime.
4) When the alarm went off it was time to brush teeth, get jammies on, go to the bathroom, kisses for everyone, then pick out a storybook.
5) Either I or my husband layed in bed with them every night and read the chosen story.
We did this until they went to kindergarden. It's a wonderful memory to create. I think it also helps them read better. They are in college now and both love to read.
The biggest issue you will probably face is her wanting to get out of the bed. If you NEVER let this occur, it should never become an issue. You have to be the one in charge for sure.
I didn't put the mattress on the floor. I don't ever remember them falling out. Let's face it, the way we learn how to stay on the bed is by hitting the floor a couple times.
Good luck to you and her and congrats on your new bundle soon to come.
(A small aside...It has cost us nearly $30,000 to send these 2 kids to college. If I was back to where you are now, I'd be socking away the college money from the get go. Start now!! )