What Causes Thick, Dark Menstrual Blood?

My 15-year-old daughter is on her period and is alarmed that the color is a very dark brown. It is very heavy and seems thick. I told her it probably was her body chemistry changing. What else might it be? She has irregular periods, but I have been attributing that to all of the sports she plays.


Dark, thick menstrual blood is old blood. Sometimes blood stays in the uterus for a few days, and when it comes out it is brown. Think about how a bloodstain on clothing looks after a few days -- it does not stay bright red. If associated with severe cramping, this brown menstrual blood may be a sign that the cervical opening is too narrow, preventing free flow of menstrual blood; the blood sits in the uterus for a while, until contractions of the uterus push it out.

If your daughter's cycles are irregular, she may not be ovulating. In this case, portions of the uterine lining may be shed erratically, creating irregular bleeding that is mistaken for a real period. These small amounts of tissue may flow out as well as when the entire lining sheds with a normal menstrual cycle. Because it may take longer for this blood to be extruded, it may look browner and thicker.

Sometimes an infection can cause menstrual blood to look -- and smell -- funny. If associated with a foul odor (remember blood itself often has a characteristic smell), tenderness or fever, this should definitely prompt a visit to the gynecologist.

by Kelly Shanahan