I hope we all now know that babies should only sleep on their backs & NEVER on their bellies. It has been found to dramatically reduce the risk for SIDS. You can do “tummy time” during the day w/ baby when you are near by to help strengthen those muscles. Here are a few links:
http://www.nichd.nih.gov/sids/
http://www.keepkidshealthy.com/welcome/safety/back_to_sleep.html