Pregnancy Week 32: Physical Development
If you have checked with your doctor and it's not preterm labor, you can blame your backache on the growing uterus and hormonal changes.
Your growing uterus shifts your center of gravity and stretches and weakens your abdominal muscles, thus changing your posture and putting a strain on your back.
Hormonal changes in pregnancy soften and loosen your joints and the ligaments that attach your pelvic bones to your spine. This can make you feel less stable and cause pain when you walk, stand or even sit for long.