To get the smooth hump that you see in newborn photos do the following:
1 - Have a very firm large beanbag
2 - Place a folded towel or blanket on the bag to the side
3 - Add another blanket covering the towel to smooth the hump
4 - Lastly add your cover blanket
You can use this set up for a lot of different poses. You just have to bring your blanket down from the backdrop stand and rotate the beanbag and repose the baby.
Examples posted in the comments