The good news is, your baby is growing. That's exactly what should be happening but it can still be tough on your back.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.another reason is hormone changes happening in your body during pregnancy. Emotional stress can cause muscle tension in the back, which may be felt as back pain or back spasms.