Before you have your period, the lining of your uterus (womb) known as the endometrium becomes thicker in preparation for a fertilised egg to embed itself. During your period, the thickened lining is shed before a new cycle can begin – your period.
Your body typically releases anti-coagulants to stop your period blood clotting as it’s being released, but when your period is heavy and blood is being rapidly expelled, there’s not enough time for these to work, so clots will form.
10 pence piece
Most women experience clotting during their periods at some time, but if you always have clots in your menstrual blood and/or these are larger than a 10p coin or a bottle top, you may be suffering from heavy periods (menorrhagia)Center for Disease Control. Menorrhagia. Last accessed Aug 2017 https://www.cdc.gov/ncbddd/blooddisorders/women/menorrhagia.html.
Heavy periods are a recognised medical condition for which there are treatments. If you have excessive clotting or clots larger than a 10p piece, you should contact your GP to rule out any conditions that might be causing an abnormal period. Read about how to prepare for your GP appointment here.