The internal organs and blood vessels inside a mother’s belly are under an extreme pressure as the baby grows bigger and bigger throughout the pregnancy. The baby's kicks to mother's bladder and the intestines are sometimes painful and uncomfortable for the mother. Apart form the pain the mother feels there are some scientific reasons explaining the benefits of sleeping on the left - 

Mother's Inferior Vena Cava (IVC)

Inferior vena cava is a large vein that slides along the right side of the spine and is responsible for streaming blood from the bottom of the body to the heart. When a pregnant woman lays on her back, the baby consistently compresses the inferior vena cava and hence decreasing the amount of blood back to the heart from the bottom half of the body. Less blood into the heart is less blood out of the heart – and that means a drop in blood pressure for mother and a drop in blood-oxygen to both mother and baby (Mother’s blood supplies oxygen to the baby). Mothers and babies can compensate for a slight change in blood output from the heart but IVC can become a risk factor for pregnant women who already have blood pressure and breathing problems. As in the condition of Asthma or Sleep Apnea (A condition in which breathing repeatedly starts and stops at night) in which pregnant women already have difficulty transferring oxygen to their bodies and their babies. When conditions like these partners with less blood flow while sleeping on your back could multiply each other in a dangerous way.

It is also said by the number of studies that sleeping on your back is associated with a great risk for stillbirth.  A recent study,  BJOG: An International Journal of Obstetrics and Gynaecology found that women who had a stillbirth after 28 weeks gestation were 2.3 times more likely to have slept on their back night before the stillbirth than with a healthy continuing pregnancy.

Why not on the right side?

It is hard to say as there are not many studies regarding the left-right side sleeping during pregnancy. A study of 155 women, published in the journal The BMJ in 2011, found a higher risk of stillbirth in women who slept on their right the night before they miscarried, but the results have not yet been repeated. But, there is no clear evidence that sleeping on the right side is not so good than sleeping on your left.  If you are having a healthy pregnancy then sleeping on your right is not to be worried of. It is essential that you get a sound sleep during pregnancy.

Research suggests that lack of sleep (less than 5 or 6 hours of sleep at night) leads to an increased risk of gestational diabetes and preeclampsia.