You can put a pillow on one side, under your back too. just enough that you are off center and 1 inch off to either side. Something about an vein or artery that needs to be off center or it could hurt one of you.
Yes, I asked my doctor last week when I am not allowed to sleep on my back anymore and she said when it makes you uncomfortable. She said your body will let you know if you shouldn't do that :)
Yes, like she said , prop your upper half up some. Like an extra pillow or stick something under the head of the mattress to lift it. I sleep on my back and I'm almost 22 weeks as well :)
Just prop yourself up a bit you should be fine. I have the same problem lol. My ob said elevating your upper half a little will prevent any possible ill side effects.... If they were to even happen.