The pain in the left lower abdomen in the third trimester can be ectopic pregnancy, and there is also the possibility of simple gastroenteritis, and some people may have stones in the urinary system, which are the main ones. So pay attention to the nature of your pain, whether it is a bout of pain or a constant pain. If it is gastroenteritis, the pain usually comes in bursts. In the case of ectopic pregnancy or urinary stones, the pain is often more intense and persistent. Ultrasound is usually recommended to make a basic determination, and blood tests are done to look for inflammatory indicators. A basic judgment is made based on the results of the blood tests, the results of the stomach examination and the ultrasound findings.