The left side of the back pain when you are pregnant is related to the enlargement of the uterus, elevated hormone levels, and exertion. 1. Enlargement of the uterus: After pregnancy, the uterus will slowly increase in size, especially in the late stages of pregnancy, the uterus will cause pressure on other organs in the body, affecting the lower back and causing pain. 2. Increased hormone levels: After pregnancy, estrogen levels in the body will increase, and after the hormone levels increase, the muscles and ligaments of the body will also show corresponding changes, and the symptoms of lower back pain will also occur. 3. Exhaustion: after pregnancy, due to the uterus increases, the center of gravity of the body will be back, this time the waist to bear the weight will increase, easy to strain the situation, in addition, if you often do some bending movements, will also cause lumbar strain, lumbar pain. Pregnant left back pain may also have other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.