more reading here
http://www.babycenter.com/0_developmental-milestones-rolling-over_6504.bc
Rolling over
Once your baby has head control, and about the same time that he learns to sit on his own, he'll learn to roll over. He'll eventually learn to flip over from his back to his tummy and vice versa, and he'll use his newfound skill to get around a bit. The incentive for those early rolls is often an elusive toy — or you.
When it develops
Your baby may be able to kick himself over, from his tummy to his back, as early as age 2 to 3 months. It may take him until he's about 5 or 6 months to flip from back to front, though, because he needs stronger neck and arm muscles for that maneuver.
How it develops
Baby on the move: Sitting
At about 3 months, when placed on his stomach, your baby will lift his head and shoulders high, using his arms for support. This mini-pushup helps him strengthen the muscles he'll use to roll over. He'll amaze you (and himself!) the first time he flips over. (While babies often flip from front to back first, doing it the other way is perfectly normal, too.)
At 5 months your baby will probably be able to lift his head, push up on his arms, and arch his back to lift his chest off the ground. He may even rock on his stomach, kick his legs, and swim with his arms. All these exercises help him develop the muscles he needs to roll over in both directions — likely by the time he's about 6 months old.
While some babies adopt rolling as their primary mode of ground transportation for a while, others skip it altogether and move on to sitting, lunging, and crawling. As long as your child continues to gain new skills and shows interest in getting around and exploring his environment, don't worry.
