@Kat919 - It's actually not usually the end of the world to lie on your back for a little while, even in late pregnancy. Yes, if you do it for a long time, or in some circumstances, it *can* interfere with inferior vena cava function, but you will feel that happening in plenty of time to get off your back.
I only bring this up because there are some exercises and stretches that expectant moms can do that help align their pelvis or get baby into a better position that require lying on your back for a few minutes. (I like the "Spinning Babies" website, but there are other sources out there.) I wouldn't want moms to be afraid to try them!