When can i start leaving my baby sleeping on her stomache?

Depends on age: It is not clear what your infants age is, but some "tummy time" is good for development from birth...But sleeping on her back leads to a 50% decrease in sudden infant death. Some infants will begin to roll over in sleep by about 4-5 months and i advise that this is ok but i advise leaving them on back as long as possible for sleep initiation.

Back to sleep: The recommendation by the american academy of pediatrics is to put babies on their backs to sleep. If your baby is able to roll front to back and back to front, she might roll herself over because she prefers to sleep on her stomach. If this is the case, you don't have to spend all night trying to roll her back. You should still keep her crib free of bedding.

Hard to say: Given that crib death is loosely defined to include the first year of life, I don't put them down on their belly for that period.But I realize that they may begin turning over by 4 months and I don't get up to monitor or try to change their eventual choice of sleep position.