Hemorrhoids may be caused by a few different factors. The most common cause of hemorrhoids is repeated straining while having a bowel movement. Straining gets in the way of blood flow into and out of the area, which results in pooling of blood and enlargement of the vessels in that area. For the same reason, a severe case of constipation or diarrhea can also cause hemorrhoids. Additionally, pregnant women may be at increased risk of hemorrhoids because of the pressure that the uterus places on these veins.