The common reasons are as follows: First, too many red blood cells in the body is the most common reason, the color of blood is mainly determined by oxygenated hemoglobin, oxygenated hemoglobin mainly exists in red blood cells, red blood cells are the main component of blood cells, when the number of red blood cells in the body is too much blood color will be relatively dark, if the red blood cells are less, the color of blood will be light. Second, there may be metabolic diseases, such as the liver’s detoxification ability may have decreased, resulting in the body’s toxins or metabolic waste in the blood stagnation will make the color of the blood increased. Third, there may be problems with the endocrine function of women, especially the sex hormones, estrogen and luteinizing hormone that regulate the menstrual cycle, hormones are very important to the regulation of menstruation, if the content or concentration in the body changes, it can also lead to abnormal blood color, volume and duration.