I ran into this with my last pregnancy in the mid third trimester and it was absolutely awful. Starting in the second trimester I made a concentrated effort not to sleep on my back but since that was my normal default sleep position pre-pregnancy I would awaken on my back having rolled at some point while I was sleeping. By the 32 weeks or so I would awaken with tingly hands and legs. To try and stop me from rolling over I sandwiched myself between pillows with only enough room to sleep on my side. Didn't stop me all that much and sleeping on my back never felt uncomfortable.
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The real scare came at about at 35 weeks when I awoke suddenly gasping out of a deep sleep. My legs had gone totally numb. I felt like the room was spinning and fading in and out. I was on my back and in a blind panic I started flailing (waking DH) who helped me roll over. After I was able to take some deep breaths and regulate my heart beat (which was way too slow) I noticed my little baby moving erratically and frantically. Not the normal "I'm gonna keep mommy awake" pattern I had been acustomed too. Luckily everything was fine (a frantic midnight call to the midwife and hours upon hours of kick counts and worries later). After that I slept on a low padded bench (with a mattress below in case I rolled off) that was not wide enough for me to sleep on my back.
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Just an example of someone who has encountered it. Its a real issue but it does not effect all pregnant women. I started training myself to sleep on my side as soon as I saw those two pink lines this time around. But my last experience left me a little jumpy.