Hello, The pills contain a hormone which reduces the amount of progesterone which is secreted your ovaries. Progesterone is the hormone that is needed to maintain the uterine lining and allows a fertilized egg to implant. Without a uterine lining, you cannot get pregnant. The emergency contraception is designed to reduce the amount of progesterone secreted which causes the uterine lining to shed. During menstruation, the thick lining of your uterus (womb) breaks away. As you menstruate, anticoagulants are released that break down thick menstrual blood before it leaves your body. During a heavy flow, blood is expelled faster, and the anticoagulants may not have enough time to break down the blood. That is when the clots form. If you are passing many thick, large clots or bleeding heavily every month, it could be caused by a health issue. Visiting your doctor will help you rule out any further problems. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Penchila Prasad Kandikattu