Menstrual blood is usually bright to dark red and may or may not have some clots. It should not be brown to black and if it is then it needs to be investigated further. The main reason why blood may appear these colors is due to degradation of the blood cells either due to exposure to air (oxidation) over time, the action of bacterial enzymes or exposure of blood to certain chemicals that may cause rapid degradation and discoloration. A brown to black vaginal bleed in a pregnant women particularly if it is accompanied by abdominal pain should be investigated immediately.

The sight of blood clots, when you have your period, can be quite alarming and you may not want to seek medical attention for such a personal ailment. In most cases menstrual clotting is normal and may be accompanied by stomach cramping.

The presence of blood clots may only be an indication of heavy periods which is known as menorrhagia. Excessive bleeding during a period may cause the formation of blood clots and this alone is not always a cause for concern unless the heavy bleeding leads to other complications such as anemia. However, if the bleeding is excessive and prolonged and you have been passing large clots too frequently, it would be advisable to consult with your doctor.