When it starts to feel bad. <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/winky.gif" style="border:0px solid;" title="Wink"><br><br>
Actually, I still sometimes lay on my back even at 34 weeks. I think though that "they" tell you to stop around 15-20 weeks. Has to do with circulation more than anything. When you get bigger, it could also cause problems with heartburn, digestion.<br><br>
More than likely, you'll begin to realize it isn't as comfortable as before, but like I said, I still sometimes wake up to find I am laying flat on my back, and sometimes even on my belly!
like the other 2 posters have said.. you will know. it wont feel very good after a few seconds/minutes. i have never not laid on my back through a pregnancy, but when it starts feeling bad i would roll over. i think that to do any kind of dammage to the baby you would have to feel really really sick yourself.
Like PP's said, you will know if you listen to your body. I'm still sleeping on my back some of the night at 24 weeks along. Lying on my side makes my pelvis hurt. No more belly sleeping so the only other option is on my back. It never makes me feel icky.
I'm only 14 weeks, and after sleeping on my back for several hours, woke this morning with numbish legs. Propping a pillow under one of my hips solved it, while still allowing me to lie on my back. Now I know I just need to relieve that particular sort of pressure, and things are fine.