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Tummy sleeping is actually more of an issue for mom than it is for baby in most cases. Your baby is sort of like an internal organ, fragile but also protected. But tummy sleeping can cause you a lot of discomfort. So basically, you'll know when to stop tummy sleeping when you can no longer tummy sleep comfortably.

Sleeping on the left side is the best for most women because it takes pressure off the back and organs and increases blood flow. But what works for most women may or may not work for you, so ask your doctor just to be safe.

You can read more about it at these sites:
http://kidshealth.org/parent/pregnancy_newborn/pregnancy/sleep_during_pregnancy.html
http://www.babycenter.com/406_is-it-safe-to-sleep-on-my-stomach-during-pregnancy_1245307.bc
http://www.americanpregnancy.org/pregnancyhealth/sleepingpositions.html

If its comfortable for you, its almost always fine for the baby and will do no harm to him/her in any way.