Changes in the color and thickness of menstrual blood are often normal. But there are a number of problems that might cause abnormal clots to form in your menstrual blood or lead to the changes in color or thickness during your period. Remember, it's important to discuss any concerns you have with your doctor. Problems that can cause changes include:
Miscarriage. Women who have miscarried may pass blood clots or gray clumps of tissue from the vagina. If there is a chance you are pregnant, be sure to check with your doctor immediately if you notice excessive bleeding or clotting. If there is a chance you are pregnant AND you notice excessive bleeding or clotting, be sure to notify your doctor. However, light bleeding and spotting during pregnancy is not abnormal.
Fibroids. Uterine fibroids are also called leiomyomas. These are noncancerous tumors that form in the uterus. Fibroids do not always cause symptoms. In fact, increasing research information suggests that most women with small "fibroid" tumors have no symptoms at all. But women with fibroids may notice greater than usual amounts of menstrual blood. If you have fibroids, you may have more clots in your period than you had in the past.
Hormonal changes. Your body relies on a delicate balance of progesterone and estrogen. These hormones regulate the production and shedding of the uterine lining. When this balance is disturbed, it can lead to the development of an excessively thick uterine lining. This thickness can contribute to more bleeding than usual. It can also cause clots in the menstrual blood when the lining is shed.
Hormone changes may occur for many reasons, including:
Recent dramatic weight change
Side effects from some medications, including steroids
Large uterus. If your uterus has been stretched during pregnancy and does not return to its original size, it may be permanently enlarged. With an enlarged uterus, menstrual blood may have time to collect and clot before it's released from the body. This could also result in a dark color or thickening of your menstrual flow.
Obstruction of menstrual blood. Anything that hinders or blocks the flow of menstrual blood from the uterus through the cervix and out of the vagina may lead to problems with clots, color, or thickness of menstrual blood. Benign polyps in the uterus may change the flow of blood during your period. The flow can also be slowed around the time of menopause when the cervical canal may become smaller as estrogen levels drop.
Adenomyosis or endometriosis. These related conditions occur when the tissue that forms the uterine lining is found in the wrong place. In endometriosis, this tissue develops outside of the uterus. In adenomyosis it grows in the muscle that makes up the uterine walls. Both of these conditions can lead to abnormal periods and heavy flow. This can increase the likelihood of menstrual blood problems such as clotting or thickness.