Upper abdominal pain can be from gastritis, dyspepsia or acid reflux, which can happen in pregnancy. You could try using two pillows, to keep your head slightly more elevated when you sleep. You could also try some simple antacids like ranitidine if you have this problem. If the pain is muscular or posture related, then you will have to adopt a comfortable posture when you sleep. You do not need to completely turn to your left, but could just tilt yourself to the left by putting pillows on the right, for support.