Backache is very common, mainly due to the increase in hormones (mainly progesterone) which necessary to maintain the pregnancy but also relaxes ligamnets and joints. Because the muscles and ligaments soften and stretch instability and discomfort of joints can result. This is further complicated by the pregnancy growing and therefore expanding, which instinctively causes you to ''tip'' slightly backwards resulting in further strain. The curvature of your spine ends up becoming much more pronounced and trapped nerves, aching muscles and ligaments can result. This should get better within a few weeks of delivery!