@nicemmcha2015, what she said... My niece suffocated while sleeping on her stomach.

the theory I heard is on your chest they'll hear your heart beat and breathing and it'll keep them breathing.

the point of not laying them on their stomachs is if they are unattended and not being checked on they can suffocate due to lack of neck control. if they are laying on your chest you're obviously going to be aware of the position of their face and if something happens you'll know if they are in destress.