Back pain is one of the common problems in pregnancy. Pregnancy hormones tend to relax the ligaments and joints of the body, in preparation of the labour process. In advanced pregnancy, the centre of gravity of the body shifts, owing to the pregnancy bump, and the curvature of the lower spine changes.
You have not mentioned whether it is lower back ache or upper.
Trying to maintain a good posture while sitting and lying down will help. Try to sit on a comfortable chair, which will support your back. You may use a cushion in addition. Lying on your left or right side, using more number of pillows may keep you comfortable.
You could try hot water bag application and diclofenac spray or ointment on your back. If these do not help, you can try taking a simple painkiller like Paracetamol. Physiotherapy on your back also may help, if not in very advanced pregnancy.