Near the end of the menstrual period, the blood can turn a dark brown or black, which WebMD states is common and not cause for alarm. What To Expect warns that brown discharge can also be an early symptom of pregnancy or indicate ovulation or perimenopause.
The most common reason for abnormal brown menstrual blood is implantation bleeding, says What to Expect. It doesn't occur in every woman, but implantation bleeding is caused by the fertilized egg attaching itself to the uterine lining roughly five to 10 days after conception. Accompanying symptoms such as bloating, acne and breast swelling are often confused with premenstrual syndrome.
If the change in color only lasts for a few days, What to Expect states that this is normal. Blood darkens as it ages and it can take several days after the end of the menstrual cycle for the body to finish expelling all the blood. However, if it happens frequently, is accompanied by cramping or lasts for longer than a week, consult a doctor.
Rarely, brown menstrual flow or discharge is a symptom of illness, according to What to Expect. In these cases, it is usually accompanied by pain, fever, odor or weight gain. Very rarely, heavy brown menstrual flow and pain can indicate cervical cancer.