There are several causes of back pain during pregnancy. They all increase back strain and result in impaction of nerve roots. One of the main causes of increased back strain during pregnancy is weight gain. During a normal pregnancy, women gain from 25 to 35 pounds. The center of gravity shifts during pregnancy. This can cause changes in posture and pain in the lower back as a result.
During pregnancy, the body produces a hormone, relaxin, which allows ligaments in the pelvis to relax before the birth process. This hormone can cause spinal ligaments to loosen, leading to instability in the spine and back pain. If women have disc protrusion or degenerative discs changes before pregnancy, these can herniate and cause low back pain during pregnancy.