Why is it important for a pregnant woman to lie on her left side while sleeping? What if she prefers to lie on the right side?

It is recommended to sleep on your left for maximum flow of blood to the fetus and ensuring that the kidneys function well enough. That being said, sleeping on your left can also help to reduce water retention. I would say it is ok to sleep on the right should you need to, but anything is better than sleeping on your back as your abdomen will be resting on your intestines and major blood vessels. This will lead to breathing problems, backaches and reduction in blood supply. http://ph.theasianparent.com/pregnancy-concerns-sleeping-on-your-left-side/