I recently read that once you are in the 2nd trimester, that it is dangerous to lie flat on your back because the weight of the uterus cuts off the circulation of oxygen and blood to the baby. This kind of sucks, because I primarily sleep on my back. So I decided to start practising.
In the same book they also said that putting a pillow between your legs helps you to stay on your side at night, so I decided to give that a try. Surprisingly, it works pretty well. Though last night I think I woke up on my back twice, though the night before I didn't at all. When I am not waking up on my back, I am waking up without an arm or a leg. My hips and shoulders aren't used to being crushed all night and so they are going numb. It is quite annoying because I hate the feeling of pins and needles in my entire arm or leg and I keep having to turn over because apparently my body cannot figure that part out without waking me up.
I am glad that I started practising now, because come the 2nd trimester I don't want to be waking up on my back or without limbs. All this practice will be worth it for my baby :)