A.G.
2-4 months is the peak time...but it is the leading cause of death among infants up till a year :(. I believe it tapers off after a year. If you are wondering when you can lay her on her stomach (because like most babies, she probably sleeps better on her stomach...), I would still put her on her back for quite some time. You can try buckling her into her carseat and see if she sleeps a little better that way (we had 2 kids that did...)