左侧!!

Your best bedtime bet is to lie on your side. Though it's less important which side you choose, the left side allows for maximum blood flow (less pressure on the vena cava) and could reduce swelling in the legs. To get yourself comfy in that position, pull out all the stops — and all the pillows. You can try putting one pillow between your legs, one under your belly, and one behind your back — or any other improvised combination that allows you to sleep like a baby. You can try a wedge-shaped pillow for support. Another option is a full-length body pillow (which, when you shift positions with it, actually provides good training for alligator wrestling).

Best intentions (and nightly get-comfortable rituals) notwithstanding, you can't always control what pose your body strikes in your sleep. If you wake up on your tummy or your back — don't worry — absolutely no harm will come to your baby. That you woke up in the first place is probably your pregnant body's way of telling you to change positions (and maybe that it's time to pee again). So (after your trip to the bathroom) just haul yourself back into position — which at this point is probably no small undertaking (relocate four pillows, swing over the legs and belly, push out buttocks, rearrange the pillows…are you comfortable yet?).

-quote from http://www.whattoexpect.com/pregnancy/ask-heidi/week-27/sleep-positions.aspx