Passing blood clots during menstruation can be normal. The amount, length and frequency of menstrual bleeding vary from month to month and from woman to woman.
However, passing large blood clots may be a sign that something's wrong. If your periods seem heavier than usual — for instance, soaking through one pad or tampon every hour for several hours or passing very large blood clots — check with your doctor.
Your doctor can determine if there's an underlying problem causing your heavy periods. Your doctor may recommend a blood test to check for anemia, a blood condition that can cause you to feel weak or tired. Your doctor may also recommend an ultrasound of your pelvis. Sometimes doctors prescribe hormonal medications to help regulate heavy bleeding.