Some clotting in menstrual blood is normal and occurs at the heaviest point of a period where the body is expelling blood at such a rapid rate that natural anticoagulants can't work; however, excessive clotting can signify a problem, according to WebMD. Any time blood clots the size of a quarter are passed, it is important to see a doctor, according to the U.S. Center For Disease Control and Prevention.
Large blood clots sometimes signify uterine-related problems, including uterine fibroids or polyps, uterine or cervical cancer, miscarriage or ectopic pregnancy, as stated by the U.S. Center for Disease Control and Prevention. According to WebMD, heavy bleeding with clots can result from hormonal changes related to menopause, dramatic weight changes, adenomyosis, endometriosis or a large uterus stretched from childbirth that doesn't return to its regular size.
Blood clots can also be a sign of bleeding-related disorders as well as non-bleeding related disorders, such as liver, kidney or thyroid disease, pelvic inflammatory disease or cancer, according to the U.S. Center for Disease Control and Prevention. This source advises that a doctor takes a history and may perform a blood test, pap test, endometrial biopsy or ultrasound to determine a cause for the blood clots.