As babies mature, some will choose to sleep on their tummies. There is nothing parents can do to prevent this and there is no point in getting up constantly during the night to move the baby onto his back. However, always put him on his back when you put him in his cot. Keep in mind that the peak age of cot death is 4-12 weeks and that, by the time most babies are able to roll over, they are past this high-risk period.