my 1st daughter started rolling onto her tummy at only 3 mo old and my pediatrician told me that once a baby has enough strength to roll over on their own, the risk of SIDS greatly decreases. it's when the baby can't physically move themselves in case of lack of oxygen that you need to worry. so as long as you are taking other precautions (no pillows in the crib, not a bunch of blankets, etc.) then your baby should be just fine. using a pacifier can also decrease the risk of SIDS, because it puts a little extra space between your baby's nose and the mattress