Yes that is somewhat true. The baby books say to try to lay on your side after about 15 weeks I believe it is.

Is it oxygen to the baby? I thought it was some major artery to the mom. My doctor told me to sleep however I was comfortable and that if there was a problem i'd feel it and move. I slept on my back until it was no longer comfy, probably up until 30 weeks or so, and I have a perfect beautiful baby.

It rests on some sort of artery or something and blocks blood flow to you and the baby. I'm with bridget tho, I slept on my back until it became uncomfortable then switched to my side.