During pregnancy your expanding uterus stretches and weakens your abdominal muscles and alters your posture, putting strain on your back.
The extra weight from your pregnancy also means extra work for your muscles and increased stress on your joints.
Hormonal changes in pregnancy can loosen the joints and the ligaments that attach your pelvic bones to your spine which can make you feel less stable (pregnancy waddle) and cause pain when you walk, stand or sit for long periods. This hormone, called relaxin, allows your baby to pass through the pelvis.