Blood spots in the underwear after each urination may be due to urinary tract infection, urinary stones, glomerulonephritis, urinary tract tumors and so on. 1. Urinary tract infection: If the urethra is infected by bacteria, virus or parasites, the urethral mucosa will be edematous and congested under the stimulation of inflammatory cells and pathogenic microorganisms, and the patient will have blood in the urine when urinating, and the blood may drip onto the underwear. 2. Urinary stones: when urinating, because the stones block the urethra and cause poor urination, coupled with the urine flushing stones, so that they move in the urethra, may scratch the urethral mucosa, so there will be a urethra with blood out of the phenomenon of blood on the underwear may be from the urethra out of the blood spots. 3. Glomerulonephritis: Glomerulonephritis can lead to increased permeability of the basement membrane due to the deposition of antigen-antibody complexes to the basement membrane, causing red blood cells to leak out with the urine, thus resulting in blood in the urine. It can lead to blood spots in the underwear after each urination. 4. Urological tumor: If there is a tumor on the epithelial cells of the urinary tract, if the surface of the tumor is eroded and bleeding, the blood will be discharged along the urethra and dripped into the underpants when urinating. There may be other reasons why there are blood spots in the underwear after each urination, and it is recommended to actively seek medical attention under the guidance of a professional physician.