Congratulations Rob, exciting times & sleepless nights ahead!
I would opt for either the 50mm or 85mm, if you only have natural light at your disposal set up next to a patio window, lay a duvet down over a couple of pillows, white sheet covering it pegged to a chair for the background. Newborns are best photographed between 6-10 days as you can pose them easier. If you want plenty of sleepy shots keep the temperature in the room warm.
One of my best baby posing items is a bean bag, ideal for getting them in the right poses comfortably & safely.
Happy shooting, you will get plenty of practise I am sure.