Convince me why I shouldn't sleep on my back?

I know basically that there is a major artery (or vein, I forget which) that runs down your back and that, when you lie on your back, the weight of the baby, after a certain point, can put pressure on that artery and decrease the oxygen levels to the baby. At least that's the way I understood it the first time around.<br><br>
But, what doesn't make sense to me is...why wouldn't my body let me know if I was doing something so dangerous? Why would I <i>only</i> be able to get comfortable...

Convince me why I shouldn't sleep on my back?