The most common cause of dark vaginal discharge is the presence of blood in the discharge. In most cases, this is normal and is not a cause for alarm. Women often have dark vaginal discharge at the end of their menstrual periods and sometimes in the middle of their period, during ovulation. It is also possible for women to bleed during pregnancy, although this may be an indication that there is a problem with the pregnancy, and the woman should be seen by a doctor. In some cases, serious infections can cause bleeding from the various organs of the reproductive system, although modern medicine can usually detect and cure these conditions before they become so serious.
It is very common for a woman to have dark vaginal discharge at the end of each menstrual cycle. This discharge is made up of the lining of the uterus, which is shed once a month if a woman doesn't get pregnant. It is normal for the color and consistency of menstrual fluids to change over the course of each period. The fluid may darken towards the end of the period because the blood it contains is older. Many women have dark vaginal discharge that ranges from a dark rust-red color to brown or even black.
Ovulation can be another cause of dark vaginal discharge. Some women experience some light spotting as the egg is released into the fallopian tube. The color of this spotting can range from red to dark brown or black, depending on how long it takes to pass through the uterus and vagina. As long as the bleeding is light, this is considered a normal condition.
Heavy bleeding that occurs between periods or any bleeding that occurs while a woman is pregnant should be evaluated by a doctor. The color of the blood that is expelled at this time may be dark or red and may indicate that there is a problem. Women with dark vaginal discharge during pregnancy should be examined to determine if the cervix is dilated, which may indicate miscarriage. Those with frequent bleeding between periods should be tested to determine if a serious medical condition is the cause.
Occasionally, after a particularly severe infection, the vaginal discharge will contain blood. This can cause dark vaginal discharge, although this is rare. In most cases, vaginal infections are diagnosed and treated before the infection has progressed to the point where it can cause bleeding.