Why is it better to sleep on your left side when pregnant?
Why does everything you read tell you to sleep only on your left side during pregnancy? One guiding principle of pregnancy is that things that are bad for you typically feel terrible. So I have my doubts about the absolute necessity of left-sided sleeping. And I know of no scientific research that says that babies do any better if their moms always slept on their sides.
The argument for not lying flat on your back comes from the fact that when you are supine the uterus lies against the vena cava, the large vein that carries blood from your body back to your heart. As the uterus enlarges beyond about 20 weeks size, the vena cava blood flow may be slowed, so less blood gets back around to the baby, leading to slowing of the baby's heart rate. Mom's blood pressure may drop as well. The good news is that just tilting to the left (by putting a pillow under your right hip) or lying all the way onto either side (right or left) can prevent vena cava compression. So there are lots of positions you can sleep in that can be comfortable and will not compromise blood flow.
And if you find yourself on your back when you wake up, don't worry too much. Moms-to-be have probably been sleeping on their backs for millennia!