[header] How to prevent large blood clots during menstruation [title] Look for blood clots. [step] One of the main signs of heavy bleeding (also called menorrhagia) is having blood clots in your flow. For this diagnosis, blood clots the size of a quarter or larger are considered to be connected to heavy bleeding. 
Check your pad, tampon, and the toilet for blood clots. [substeps] Blood clots will look like regular menstrual blood, except they will be more solid, almost jelly-like.