Causes of Back Pain in Pregnant Women:

Pregnancy back pain typically happens where the pelvis meets your spine, at the sacroiliac joint.

There are many possible reasons why it happens. Here are some of the more likely causes:

• Weight gain : During a healthy pregnancy, women typically gain between 25 and 35 pounds. The spine has to support that weight. That can cause lower back pain. The weight of the growing baby and uterus also puts pressure on the blood vessels and nerves in the pelvis and back.
• Posture changes : Pregnancy shifts your center of gravity. As a result, you may gradually — even without noticing — begin to adjust your posture and the way you move. This may result in back pain or strain.
• Hormone changes : During pregnancy, your body makes a hormone called relaxin that allows ligaments in the pelvic area to relax and the joints to become looser in preparation for the birth process. The same hormone can cause ligaments that support the spine to loosen, leading to instability and pain.
• Muscle separation : As the uterus expands, two parallel sheets of muscles (the rectal abdominis muscles), which run from the rib cage to the pubic bone, may separate along the center seam. This separation may worsen back pain.
• Stress : Emotional stress can cause muscle tension in the back, which may be felt as back pain or back spasms. You may find that you experience an increase in back pain during stressful periods of your pregnancy.