I hope your talking about trading to an M3 sedan. I have a 2dr E36, and at first it's fine for the baby. Once the child is about 1yo.....different story getting them in and out of a 2dr car.
Congrats by the way. I have a 7mnth old daughter. Enjoy every bit of the first year. You won't regret it. Oh ya, you should "try" to enjoy the pregnancy too.