There are the following differences between menstrual blood and abnormal vaginal bleeding: 1. Menstrual blood is non-clotting, and no clots will be seen among the menstrual blood, whereas abnormal bleeding tends to clot, and clots will form.2. Menstrual blood is regular, whereas abnormal bleeding is not, and basically, you can make a preliminary judgment based on the projection of the time of your last menstrual period as to whether it’s a menstrual period or possibly an abnormal bleed. 3, menstrual blood is dark red, while abnormal bleeding is sometimes darker and blackish in color, and sometimes it may appear to be bright red in color. In addition, once irregular vaginal bleeding occurs promptly go to the hospital for examination.