Other times the amount of blood is invisible to the naked eye and is called microscopic hematuria . In either case, whether microscopic or gross, there are a lot of reasons why a person can get blood in her urine. One of the most common causes of hematuria is a urinary tract infection or UTI . A culture of the urine to look for bacteria will determine if that is the cause. Other common but harmless conditions are irritation of the urethra, the opening where the urine comes out.
Masturbation is another reason.
Menstrual bleeding should be considered in young girls who have entered puberty. In these conditions sometimes the blood is visible to the naked eye and sometimes it is not. Also, exercise can sometimes cause hematuria and your doctor may test for this by getting a first voided sample in the morning before your child has gotten out of bed. If the cause of the bleeding is not easily identified, further examination of the urine with a microscope can help sort out the cause. They can also determine the amount of calcium and cysteine in the urine by requesting special chemical analyses of the urine.