Apparently it is not only your artery that circulation can be cut off to, but some circulation to the baby can also be affected because when you lie on your back, everything inside the womb sorta floats more to the back, and squishes bits that are not meant to be squished (very acurate terminology I am sure!). Don't forget you wouldn't be able to feel if there was some kind of circulation to the baby cut off. I don't sleep on my back even though at 20ish weeks that is all I want to do, I am so uncomfortable on my side, I am normally a tummy sleeper