The anterior vena cava brings blood back to the woman's heart, it really has little to do with blood/O2 supply to the baby. The reason they recommend the left side is to maximize kidney function through the night (they are on your right side, sort of). I have heard more often then not, to simply listen to your body. If you experience dizziness or shortness of breath when you're ying on your back, then roll onto one of your sides. But if you are not sleeping at all, perhaps a pillow under your right side might be just the trick. I'm nearly 24 weeks with B#3 and I'm still reading on my back and sleeping on my front with a hip hitched. My MW tells that anything to maximize health and happiness, including good sleep when you're pregnant!