Four months pregnant when the waist will appear pain, the main following reasons: 1, weight gain, standing posture is not right will also cause changes in the lumbar spine load, resulting in increased lumbar spine load, lumbago, it is recommended to change to flat shoes after pregnancy. 2, the body’s estrogen and progesterone levels rise, so that the whole body of the ligamentous tissues are loose, in preparation for childbirth, lumbar intervertebral disc ligament began to relax. Under the action of gravity may load will cause excessive protrusion of the intervertebral discs, which will compress the nerves. 3, usually less exercise, bedridden time may be longer, women’s lumbar and dorsal muscle muscle strength itself is not strong enough, after bedridden strength will further decline, so the cause of lumbar pain. 4, other factors, the ureter is compressed, fluid in the kidneys, etc.. If you can’t relieve the pain by yourself, it is recommended to consult a doctor in the hospital.