The most common cause of dark vaginal discharge is the presence of blood in the discharge. In most cases, this is normal and not a cause for alarm. Women frequently have dark vaginal discharge at the end of their menstrual periods and sometimes halfway between periods, during ovulation. It is also possible for women to bleed during pregnancy, though this may be an indication that there is a problem with the pregnancy, and the woman should be examined by a doctor. In some cases, severe infections can cause bleeding from the various organs of the reproductive system, though modern medicine is usually able to catch and cure these conditions before they become this severe.

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 does not become pregnant. It is normal for the color and consistency of menstrual fluids to change over the course of each period. The fluid may become darker towards the end of the period because the blood in it is older. Many women have dark vaginal discharge that varies from a deep rust-colored red 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 travel 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 these times can be dark or red and can be an indication that there is a problem. Women with dark vaginal discharge during pregnancy should be examined to find out whether the cervix is dilated, which can indicate a miscarriage. Those with frequent bleeding between periods should be examined to determine whether a serious medical condition is the cause.

Occasionally, after a particularly severe infection, vaginal discharge will contain blood. This can cause dark vaginal discharge, though it is infrequent. In most cases, vaginal infections are diagnosed and treated before the infection has progressed to the point where it could cause bleeding.