Clots are the natural result of the body functioning well. They are a natural way of controlling bleeding. When menstrual bleeding is heavy, clotting tends to occur. Experiencing menstrual clots can be upsetting. If the clots are large they can be painful and cause cramping as they pass through the cervix: the very top of the vagina.
The following is a brief explanation as to why menstrual clots (often large clots) are formed. All blood contains a clotting factor. To enable menstrual blood to flow freely from the uterus and leave the body without clotting, the uterus produces an anti-clotting agent: plasmin. However, when menstrual loss is heavy, the anti-clotting agent may not be adequate for the menstrual period and the blood is likely to form clots. Additionally, if menstrual blood accumulates faster than the body’s ability to transfer it out of the uterus, clots can result.
More specifically . . .
Fibrin is a protein that creates a blood clot by forming a network of trapped red blood cells and platelets.
Plasmin is an enzyme that digests or breaks down fibrin. In general, menstrual blood clots form when there is a slight imbalance between fibrin (builds) and plasmin (breakdown).