I have asked this question of a lot of people since our cot is quite small and I didn't know whether to get a bigger cot or just a bed.
The consensus seems to be - it is really up to you and your family and how ready your child is. You might be thinking ahead a bit too much, worrying about regression in your daughter when you're not yet pregnant. If your daughter is sleeping through the night, is happy in the cot and there's no reason to move yet, then I would say wait until she gets too big for it.
Here are some reasons to move to a big bed that I have heard:
* Toddler is physically too big for the cot (we are getting to this stage)
* Toddler climbs out over the bars and onto the floor, putting herself at risk by falling from a height.
* New baby needs the cot.
Here are some reasons that you might want to wait:
* Toddler sleeps erratically and usually wakes up at night, wanting company. If this is the case, a bed will only mean she will hop out of bed instead of standing shaking the bars. She could get into mischief in her room, or constantly come out to find you in the living room or your bedroom. Not that that is a problem for everyone, it depends what you can handle.
* Can't afford the expense of a new bed, or want to wait before buying one.
Hm, can't think of any others from memory but that first point is a significant one. That was what made me realise my 18 month old is not really ready to be in a bed without bars just yet.
In terms of normal age ranges, I have heard people moving their baby at 12 months into a big bed, and also others staying in a large cot until 2.5 or 3, so there is a big variation.
With regards to sharing a room, it can be done. One option is to have the new baby share your room for the first 3 -6 months, in a moses basket or other small cot. That gives you a little more time for the older child to grow up just that little bit more. Then once they're in the same room, I think it will inevitably happen that there is some waking up of both the toddler and baby, but you may also find that both are a little more settled knowing they have company through the night.