According to WebMD, brown menstrual blood is usually a normal color change that occurs towards the end of a period. It could also be a sign that blood is old, and that it is not being discharged from the body quickly.
WebMD suggests that dark brown menstrual blood is rarely a cause for concern. However, if periods are unusually heavy for an extended period of time, a trip to the doctor is advisable. Excessive blood loss could eventually cause anemia. Side effects of anemia include fatigue and weakness. Individuals who have dark and heavy periods and have noticed an increase in fatigue should consult a medical professional.
According to WebMD, there are several other medical issues that could be responsible for dark menstrual blood. An enlarged uterus gives blood space to collect and clot, which leads to darker and thicker blood. If the menstrual blood is being obstructed for any reason, this could also lead to dark menstrual blood. Potential obstructions include benign uterine polyps and diminished cervical canals that indicate menopause. Adenomyosis and endometriosis can also affect blood flow. These conditions, which happen when the uterine lining tissue is in the wrong place, can lead to a variety of menstrual blood irregularities.