The cause of clots in menstrual blood is due to the lack of anticoagulants, which occurs during times of heavy flow, according to WebMD. Minor clotting is common, but if a woman experiences large or excessive clots, she should see her doctor.
Many women have clots during their period, and a normal clot is usually dark or bright red in color, explains WebMD. Most of these clots appear during the heaviest day of a woman's period, and many clots in the blood can make it seem more dense or thick than usual. The body naturally produces anticoagulants to keep the blood from clotting, but when the blood comes too quickly, the anticoagulants do not have time to work, which helps the clot form.
Blood that is thick and heavy is not usually a cause for concern, but periods that are heavy regularly may require medical assistance, states WebMD. Though many women believe that heavy periods are normal, over time, this can lead to conditions such as anemia, which may cause fatigue and weakness. If a woman feels that something is wrong with her period, she should see a doctor. The doctor starts with a check of blood counts and then moves on from there to diagnose the condition.