Menstrual Blood Problems: Clots, Color, and Thickness

If your menstrual blood varies in color and consistency throughout your monthly period, it's likely that it's perfectly normal. There are times, though, when changes in color, thickness, or clotting may indicate a problem.

You might feel embarrassed asking your health care provider about menstrual blood problems, but it is important.

What happens during a menstrual period, and how long does it last?

During your menstrual cycle, the lining of your uterus thickens to get ready for pregnancy. Then, if you are not pregnant during your period, your body sheds the uterine lining along with blood. The amount of blood and fluid lost is usually between 4 and 12 teaspoons each cycle.

The average menstrual cycle lasts 28 days (counting from the first day of bleeding to the first day of the next cycle). For some women, though, cycles can be as short as 21 days. For others, they can be as long as 35 days.

A normal period lasts between two and seven days. The average length of time for a period is three to five days.

Are clots and thicker menstrual blood unusual during a period?

Many women have clots in their menstrual blood from time to time. The clots may be bright red or dark in color. Often, these clots are shed on the heaviest days of bleeding. The presence of multiple clots in your flow may make your menstrual blood seem thick or denser than usual.

Your body typically releases anticoagulants to keep menstrual blood from clotting as it's being released. But when your period is heavy and blood is being rapidly expelled, there's not enough time for anticoagulants to work. That enables clots to form.

If you have excessive clotting or clots larger than a quarter, you should see your health care provider to rule out any conditions that might be causing an abnormal period.

Are darker colors and thicker flows normal in menstrual blood?

You may notice that your menstrual blood becomes dark brown or almost black as you near the end of your period. This is a normal color change. It happens when the blood is older and not being expelled from the body quickly.

Temporary thick, heavy flow isn't necessarily cause for concern. However, regular heavy periods justify a trip to the doctor to check your blood counts. Many women become accustomed to heavy periods, considering them to be normal. Over time, though, the excess monthly blood loss leads to anemia, potentially causing weakness or fatigue. If you ever feel something's not right with your period, see your health care provider.


What causes menstrual blood problems?

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.
  • Thyroid abnormalities
  • 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:

  • Menopause
  • 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. Reasons for an enlarged uterus include fibroids and adenomyosis, or cancer. An enlarged uterus from multiple pregnancies is normal and does not cause heavy menses.
  • 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.


How are menstrual bleeding problems diagnosed?

Depending on your symptoms, your doctor may order a variety of tests to determine the reason for menstrual blood problems. These may include:

  • Vaginal ultrasound. This test uses sound waves to take a picture of the inside of your pelvis, uterus, and ovaries.
  • MRI. This non-invasive procedure can provide an image of growths, such as fibroids, that may be contributing to your menstrual bleeding problems.
  • Blood work. Your doctor may order blood tests to determine whether your blood is clotting correctly. The tests will also help ensure that you are not suffering from anemia, an iron deficiency that can result from loss of blood or a clotting disorder.
  • Biopsy. In this procedure, your doctor will remove a small tissue sample from the lining of your uterus for analysis.
  • Dilatation and curettage. In this procedure, your cervix is dilated and a surgeon scrapes off the lining of the uterus and cervix. This can be used to help alleviate excessive bleeding or obtain tissue samples for analysis.

When should I see a doctor for menstrual bleeding problems?

Menstrual bleeding problems are rarely serious. Significant blood loss can occur over time, though, going unnoticed because it's so gradual. See your doctor if you experience any of the following:

Anemia can be diagnosed with a simple blood test. Iron pills supplements result in improvement in most women.

WebMD Medical Reference Reviewed by Nivin Todd, MD on September 08, 2016



The National Women's Health Information Center: "Menstruation and the Menstrual Cycle."

Children's Hospital Boston, Center for Young Women's Health: "Big clots of blood/Spotting of blood."

The American College of Obstetricians and Gynecologists: "Menstruation."

Center for Endometriosis Care: "Menstrual Clots: What Do They Mean?"

Cedars-Sinai Health System: "Uterine Fibroids."

© 2016 WebMD, LLC. All rights reserved.