During your first 12 weeks, you may feel sleepy and want to take a nap. First, of course, you must get the rest you need, so sleep in whatever position feels most comfortable for you, including on your back or front.
From 13-28 weeks, you will feel the baby's weight more, which at times can feel uncomfortable and put pressure on specific areas of your body. This is the time when a soft mattress will benefit you most, while your more prominent bump means the suggested position to sleep in is on your side, as sleeping on your back will make your bump press against the main blood vessel, which can cause you to feel a little faint.
During 29-40 weeks, your lower back may be a source of pain as it supports the increasing weight of your baby. To reduce this pain, proceed to sleep on your side and with a medium firmness pillow under your head, between your knees and below and around your bump to ease pain and take pressure off your hips and muscles.