Is sleeping on your back bad for baby?

Cyndi • I am married to a truly amazing man who I am proud to call my husband and best friend. We are expecting our 1st baby!
Im 30 weeks and I always try to sleep on my side but when I wake up, I usually find that I have shifted positions in my sleep and am lying flat on my back. A friend told me you shouldn't sleep on your back while pregnant. Why not?