baby sleeping position

how can i keep my little on her back while in bed? My dd keeps rolling over onto her belly today. They say it's unsafe and to leave her on her belly because of sids.

[PetiteWonder]

Im not sure if this is true but from what I was told a while ago, if your baby is able to roll onto her belly then it is safer for her to sleep that way....  I was told its still a good idea to encourage them to sleep on their back by putting them down like that but not to stress yourself out if she is shifting her into that position...  

Mlward introduced me to this thing called a Woombie, that i am using because my DD flings her arms around and wakes herself up.. but it is designed to promote back sleeping.. You could try it..  it seems to work well as we were sleeping together last night and generally if i put her on the mattress beside me instead of on me, she rolls on her side but with this thing on she didnt move at all.