yes it’s true, especially during last trimester. this is because the weight of the baby presses on vital blood vessels that transport oxygen to baby. it’s ok if you roll onto your back in the middle of sleep, just be sure to sleep sideways at the start of the night and if you wake up, just lie back onto the side. depends on the risk wanna take? it’s not easy and i hated it, but for the sake of the pregnancy i did it. my sleeping position is always left now