Is it better to lay a baby on their back or stomach?

More information on the "Back to Sleep" campaign:

• Wikipedia
• NICHD Back to Sleep Campaign

It seems to be effective:

Current practice is to put infants to sleep on their backs as it is thought to reduce the chance of SIDS. Once the baby has mastered rolling over, should he/she roll onto his/her stomach during the night, then that's OK (no need to flip them back).

This article about the Back To Sleep Campaign is quite informative.

The question is usually raised because parents are concerned about SIDS. In addition to what's already been said, the American Association of Pediatrics no longer advises side sleeping as an alternative to back-sleeping. They are also recommending that room-sharing (having a baby's crib in the parent's room) helps prevent SIDS for the first 6 months of life.

My daughter slept in a sleep positioner on her back between us in our king sized bed for her first 2 months, and then didn't care for the positioner anymore, but continued to sleep on her back (without blankets).

I love the chart that ceejayoz posted.

Incidentally, there is also an interesting relationship between sleeping independently and the risk of SIDS. Another interesting chart:

The correlations shown in the graphs in the other answers are significant. However, the causation is not well understood. Additionally, each baby is different.

Our daughter would not sleep on her back. After rocking her to a good deep sleep, we would set her down in her bassinet/co-sleeper right next to our bed, and she would wake up crying soon thereafter. We then would pull her into bed with us, where she slept in a positioner between us or in other positions in our arms, on her side, and on her stomach on my chest.

One night, we tried setting her down in the co-sleeper on her stomach, and she slept 8 hours straight. She has slept that way since. I tend not to worry about her being able to turn her head to breathe, because she has had good head control since 2 months.

I would try to follow the guidelines from the American Association of Pediatrics, but as always, remember they are guidelines and be flexible for how things apply to your baby and your particular situation.

My hospital had us prop both babies up on their sides - either with positioning blocks or a rolled up blanket. If neither were available, then we were told to put them on their backs.