In fact, the best positions are the side lying ones – and the left side is the best of all. This is because on the left side you will be keeping the pressure off your liver and it improves blood flow for both mum and baby. Side sleeping also prevents swelling, makes the heart’s job easier, and keeps the baby’s weight from appling pressure to the blood vessels in your legs and feet.
Of course, you should stop sleeping on your stomach when the expansion of your growing belly begins to make the position uncomfortable. As for ceasing to sleep on your back, in your second trimester you should begin sleeping on your side to avoid circulation problems. The concern is not much of a problem during the first trimester as your baby’s weight will not affect your circulation overmuch.
There is no firm guideline as to when to begin your side sleeping. However, if you prefer to be cautious the second trimester is the best time to start.