Matilda transfered into her big girl bed at 16 months because she was climbing in & out of the cot.

It was very very hard to do. We spent a few weeks where it took 2 hours or so to get her to sleep and we would have to sit in her room & repeatedly put her back to bed. She's gone through phases of sleeping really well and not in it but she isn't a good sleeper at the best of times.

I would definately say start trying at least 2 months before the baby comes. Maybe a month of day sleeps first and then attempt the night ones. Goodluck!