The pain in the lumbar region when pregnant women sleep flat in the third trimester is due to the disruption of hormonal secretion in the body after pregnancy, which leads to a decrease in the tension of the soft tissues, muscles and ligaments in the thoracic and lumbar regions. It is also possible that some pregnant women have mild thoracic and lumbar disc herniation before pregnancy, which may have no clinical symptoms. With the increase of fetal size, the lumbar region compensates for the posterior protrusion, which leads to a series of symptoms of thoracic and lumbar pain, including sciatic nerve pain, after the disc herniation. Some pregnant women with pre-eclampsia have symptoms that are not abdominal pain but lumbar pain, which also need to be taken seriously. Generally, after the appearance of such symptoms, short-term observation, local massage and baking lamp are given to promote local blood circulation and relieve pain, and most pregnant women can get better relief after conservative treatment. If the relief is not obvious or even if the pain worsens, you need to go to the hospital for examination as soon as possible to avoid misdiagnosis or missed diagnosis.