Just found this on a baby health website -

The best position for increased blood flow to the baby is laying on your left side. However, you needn't be upset if you awaken during the night and you are laying flat on your back. At five months there is little concern because your baby and uterus are not very heavy. You certainly may sleep on your stomach if you are comfortable, or in a semi-seated position. Your most comfortable position will change throughout pregnancy, so be prepared to experiment using many pillows to help you find just the right position.