The main causes of lower abdominal pain in mid-pregnancy are as follows: 1. paroxysmal hidden pain or colic in the lower abdomen caused by uterine spasm and fetal movement; 2. gastrointestinal-related diseases, such as enteritis, intestinal polyps, intestinal spasm, etc. causing paroxysmal pain or colic in the lower abdomen, usually accompanied by diarrhea or changes in stool properties; 3. urinary tract infections, such as cystitis, ureteritis, nephritis, etc., which are mostly accompanied by urinary tract irritation symptoms, such as frequent urination, painful urination and changes in urine color, and can be diagnosed with the aid of abdominal ultrasound, routine blood and urine tests.