What can cause large blood clots during menstration?

Yes, but ...: During menstruation, it is normal to pass some smaller clots (say, dime sized). However, if you are bleeding profusely or passing large clots, consider visiting a doctor for an evaluation of your heavy menstruation. Hormones, fibroids, polyps are common causes. A pelic exam and ultrasound are the next steps. Treatment options will depend on your future fertility plans.