20 months is an excellent time to move her, if you're going to put her in a toddler bed do it NOW, so she doesn't associate the move with the new baby, which could lead to resentment.
Be aware that most toddler beds are safe up to 50 pounds, (put one up recently and read the warnings on every model I looked at) so if you lay with her in the bed it could weaken the support. I've heard of moms who went to sit or lay on the beds with their child and they broke, quite a dangerous situation with you being pregnant. Of course, if you're using a twin size bed this wouldn't apply.