For over 20 years, pediatricians in the United States have recommended that infants sleep on their backs. One of the questions that parents frequently ask is what they should do if their baby starts rolling over before 6 months of age.
Although babies occasionally roll over in the first few months, determined rolling isn’t learned until 4 months or later. If a baby rolls over at night, most doctors (this one included) don’t recommend putting the baby on her back again. This reason for this is simple. If you turn the baby on her back, she will invariably roll to her stomach again. If you do this repeatedly throughout the night, no one will get a good night’s sleep, which could lead to other dangers, i.e., car accidents.
So while I would still recommend putting your baby to sleep on her back, I would leave her alone if she rolls to her stomach.