There are two common causes of lumbar pain on one side in the third trimester: first, after pregnancy, the uterus and surrounding tissues are filled with a large amount of blood to supply the fetus with nutrition, while the weight of the fetus itself will increase, resulting in a heavier pelvis, the uterus will twist laterally during pregnancy, the body’s center of gravity moves forward, the spine and lumbar muscle load increases, causing pain on one side of the lumbar region, which can be relieved by resting in bed; second, pregnancy The abdominal circumference does not increase excessively in the third trimester, so women will still maintain their original sitting posture, such as crossing their legs, and their bodies will be used to twisting movements, which will lead to excessive load on one side of the lumbar muscles and eventually lead to strain pain on that side of the lumbar muscles, which can be relieved within a short period of time through bed rest, massage and local heat application.