A newborn baby's head is a third of their entire weight, which is very heavy. Not only that, a baby has no control of their neck muscles, which means that if you allow a baby's head to flop too sharply in any direction you COULD cause great harm to the delicate structures inside, like the esophagus and windpipe.
For that reason, I photograph the 'froggy pose' as a composite image. Which is 2 or more images combined, to make 1 final image. You can see mom's hands here in the top images, safely supporting her son's head.
www.heidiglibotaphotography.com