There are many reasons why stools have a concave shape, such as hemorrhoids, polyps in the intestines, tumors and other causes. 1. Hemorrhoids: Various types of hemorrhoids may cause tortuous veins in the rectal-anal veins, and the stool is squeezed by the hemorrhoids during defecation, which leads to concave shape of the stool. It may also cause anal swelling, pain, blood dripping from the stool, and lumps outside the anus. 2. Intestinal polyps, tumors: intestinal polyps, tumors and other occupying diseases, may squeeze the stool, resulting in concave shape of the stool phenomenon. It may also cause abdominal pain, bloating, diarrhea, bloody stools, pus and blood stools and other symptoms. It is recommended to consult a doctor for timely treatment of concave stools.