Rose,
When my oldest child was born, they actually had just changed the laws, for your child to sleep on their bellies, and not the back.
And now they say sleep on your back and not your belly.
I honestly believe when it comes to Sids, there is no PERFECT way.
If your kids vomit at night on their backs they could choke to death accidently.
If they are on their bellies they can also Drown in their own vomit, After all their faces are in the puke,
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My OWN theory is to use one of those triangles and keep them on their sides.
This is the way I have Always done it.
At some point they either roll on the stomach, or back.
It can't be avoided. they eventually start to move.
So Do what you have been doing and Keep a very close eye on her.
Work with her every day to build her little baby muscles,
and help her learn to roll over, have tummy time to build stomach muscles, and stand her on her legs , pull her to a sitting position using her arms,
it all helps her get strong
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In addition my boys always had sinus drips that made them cough at night.
So I used a pillow behind their backs
M