Blood clots in menstrual flow result from heavy bleeding. Normally, special anticoagulants are released by the body during menstruation to keep menstrual blood from clotting during its release. Heavy flows do not allow the anticoagulants enough time to keep clots from forming, so blood clots form, according to WebMD.
Many women experience clotting during menstruation, at least on occasion. Clots can be dark red or bright red, and they are typically shed during the heaviest days of the cycle. Clots present in the flow may cause the flow to appear denser or thicker than usual. Although usually not an indicator of a serious problem, excessive clotting and clots that are larger than a United States quarter in size should be reported to a healthcare provider who can rule out any potential reasons for abnormal periods, notes WebMD.
Some of the causes of passing large clots and experiencing heavy bleeding include uterine-related problems, including uterine fibroids or polyps, cancer, and use of certain birth control methods, notes the U.S. Centers for Disease Control and Prevention. Other possible causes include an ectopic pregnancy or miscarriage, hormone-related problems and platelet function disorders. Thyroid disease, kidney and liver diseases and other non-uterine cancers can also cause this type of bleeding.