Most bleeding from the menstrual period is just from shedding lining tissue
The shed tissue does open tiny vessels on the surface of the uterine lining that causes the bleeding with the tissue that is shed
Those vessels seal like any vessels when we get a cut, through a physiological cascade of clotting factors.
For those born with deficiencies of one or more of those clotting factors, the menstrual bleeding can be very heavy. These conditions can include a platelet problem called thrombocytopenia, a deficiency of factor VIII type of hemophilia, Von Willebrand’s disease or other coagulation disorders.
Adults can acquire some of these problems, but usually they are picked up in teen years after the first few periods. But women who are older are less likely to need these tests, and other causes for heavy periods should be evaluated.
Tests such as a CBC, a prothrombin time, an activated partial thromboplastin time, and clotting factor tests might be needed. Hematologists are specialists who can help you determine what tests you need.
If you have had heavy periods from the beginning, when you were a teen, you should consider getting the tests to determine if you have deficiency of one or more clotting factors.
Knowing your blood genetics can be medically important for more than just periods.