As your uterus becomes heavier, the strain increases on your muscles, joints and ligaments, especially in the back and pelvis area. Lots of expectant mothers arch their back more as their belly grows, which puts extra strain on the lower back muscles. You may notice it as a pain in your back that gets worse during the latter part of your pregnancy.
If you have back problems, taking short rests may make you feel more comfortable and help you make it through the day. Lying on your side, with your legs bent and a pillow between your legs and feet, usually feels nice and relaxing. The right type of exercise can prevent back pain because strong muscles are more able to cope with the increased strain.
Source: - Lärobok för barnmorskor. Faxelid, E. Course literature 2001. - Myles Textbook for Midwives. Fraser, D. och Cooper, M. Churchill Livingstone 2003.