During pregnancy ligaments become softer and stretch to prepare you for labour. This can put a strain on the joints of your lower back and pelvis which can cause backache. As the baby grows, the hollow in your lower back may increase and this may also cause backache.
TO AVOID BACKACHE:
A firm mattress can help to prevent and relieve backache. If your mattress is too soft, a piece of hardboard under its length will make it firmer.
Make sure you get enough rest, particularly later in pregnancy.
If your backache is very painful, consult the pre natal exercise coordinator. She will be able to give you some advice and suggest some helpful exercises.