Your belly will be getting bigger by the day and, as a result, you may find it increasingly difficult to get comfortable when you're lying down, especially during the night.
You should avoid sleeping on your back in the second half of your pregnancy, so start practicing some new positions now. This is because the weight of your uterus will press on the major veins that return blood to your heart, which may result in dizziness, low blood pressure, and possibly a reduction in blood flow to the uterus. Ideally, lie on your left side (although it will do you or your baby no harm to lie on your right side) since this is actually good for you and the baby. It improves blood flow to the placenta and helps your kidneys eliminate fluids and waste products. Don't worry if you wake to find you're lying on your back: just roll onto your side and support yourself with pillows if necessary.
It's fine to lie on your front if you prefer (your baby is safely cushioned in the amniotic fluid), but the bigger you get, the more difficult this will become.