Lens for newborn photography sessions

[crissyanon]

I looking to purchase some new lens to use in newborn shoots and was wondering which size I should be looking at. I would like to get prime lens if I could. I currently have a Nikon D3100 with a 35mm 1.8. I was also wondering with my 35 mm how far back should I be from the baby to get a decent picture. I don't want to distort the image by being to close up. I know if I am further back I can always crop the image to get a closer shot.