Quoting Jenna+1:" I layed on my back till the day my son was born! I did t have any issues. I think they tell you not ... [snip!] ... on some vein or something and cause you to pass out and even die if no one is there to turn you. But I'm not 100% sure on that."It is true. There is like a blood vessel back there that can get pinched off and I swear it makes you feel like your blood sugar is dropping. I had that issue all the time so it was especially difficult when I was getting my ultra sounds until the tech figured out what was going on and she was able to tilt the table so that wouldn't happen.