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/

For me, I prefer to lie on my back instead of side lying during my pregnancy as I feel more comfortable to prop up both of my legs using pillows below to ease my leg cramps. It is totally up to individual on your sleeping postures and don't feel stressed over it.

As Stephanie mentioned, it's advisable as it helps ensure maximum flow of blood to your baby. Also personally I feel that it helps ease of heartburn and I slept better that way.

thanks