12-year-old girls generally enter prepubescence, some children enter puberty, relatively speaking, the urethra of girls is relatively short, so the perineum if personal hygiene is not good, it is easy to occur infection, may also go out to public places to shower, or underwear is not disinfected, especially with adults to share bathroom supplies, may occur urinary tract infection. In this case, you will see yellow discharge on your underwear, so you must go to the hospital in time to check if there is a urinary tract infection or perineal infection and see if there are bacteria in the discharge. In addition, there are no signs of fever, and you can even check the urine routine and do a urine culture test to determine whether there is a urinary tract infection. Once inflammation is present, aggressive anti-infective treatment should be given.