According to WebMD, chronic back pain can be an early sign of pregnancy. This back pain is generally caused by the hormonal balance changes that occur as a result of pregnancy.
BabyCenter states that during pregnancy, a woman's uterus expands. This causes a woman's center of gravity to shift, which gradually changes her posture and puts additional strain on the back. Additionally, the uterus may end up pressing against a nerve which can cause additional pain in the lower back region. Up to 75 percent of pregnant woman experience back pain at some point during pregnancy, and the pain can sometimes persist even after the baby arrives.