Baby Sleeping on Tummy [BC]

My baby is going to be 7 months this week and I've started putting her into her crib.

I've watched the Nanit clips the next morning and for the last 2 nights she's been sleeping on her tummy.

There's no way for me to prevent this unless I stay up and flip her back each time she goes on her tummy.

Is this okay??? I thought they should be sleeping on their back till they're 1.

Stressing me out!!