What does it mean if your blood clots are black?

September 29, 2025 •

5 min read

Approximately 90% of all blood clots are typically a deep red color, yet some appear dark brown or black. This color change can be a natural and normal part of your menstrual cycle, but in some cases, it warrants closer attention to determine the cause of your black blood clots.

Quick Summary

Black blood clots, especially during menstruation, often indicate older, oxidized blood leaving the body, particularly common during a slow flow at the start or end of a period. While this is usually harmless, if accompanied by pain, a foul odor, or persistent irregularity, it could signify an underlying health condition.

Key Points

Oxidation causes darkening: Black blood clots often signify older blood that has taken longer to exit the uterus, oxidizing upon exposure to air.
Slow flow at cycle ends: It is often normal to see black clots at the beginning or end of your period, when the flow is typically lighter and slower.
Monitor for other symptoms: Black clots with severe pain, a foul odor, fever, or excessive bleeding could signal an infection or underlying issue.
Fibroids can cause clots: Uterine fibroids or polyps can obstruct blood flow, leading to blood pooling and forming darker, thicker clots.
Check for infections: Infections like PID can cause dark discharge along with other symptoms like pain and fever, requiring prompt medical treatment.
Miscarriage possibility: In rare cases, dark discharge can be a sign of early or missed miscarriage, and medical advice should be sought if pregnancy is a possibility.

Understanding the Color of Blood

Menstrual blood can vary in color and consistency throughout your cycle, from bright red to dark brown or even black. These variations are often influenced by the time it takes for blood to exit the body, a process that determines how much oxygen the blood is exposed to. The exposure to oxygen causes oxidation, which naturally darkens the blood.

The Role of Oxidation in Blood Color

Oxidation is the primary reason behind the color of older blood. When blood is exposed to air, the hemoglobin in the blood reacts with oxygen, causing it to change from a bright red to a dark, brownish-black color. This is the same reason why a cut or bruise on your skin darkens over time.

What Normal Black Blood Clots Mean

Typically, seeing black or very dark brown clots at the beginning or end of your period is not a cause for concern. At these times, your menstrual flow is slower and lighter, meaning the blood has had more time to oxidize while waiting to be expelled from the uterus. It is simply older blood, part of the natural process of shedding the uterine lining.

Potential Medical Causes for Black Blood Clots

While often benign, black blood clots can sometimes indicate a more significant health issue, especially when accompanied by other symptoms. It is essential to be aware of these potential causes and to consult a healthcare provider if you have any concerns.

Hormonal Imbalances

Fluctuations in hormones, particularly estrogen and progesterone, can affect the thickness of the uterine lining and how quickly it is shed. Conditions like polycystic ovary syndrome (PCOS) can lead to irregular or skipped periods, causing the uterine lining to build up over a prolonged period. This older, thicker lining can be released as darker blood and clots when the period eventually occurs.

Uterine Fibroids and Polyps

Uterine fibroids are noncancerous growths that develop in the wall of the uterus, while polyps are smaller, noncancerous growths on the uterine lining. Both can interfere with the normal shedding process and blood flow, leading to heavier and more irregular bleeding with darker, clotted blood. The growths can block the outflow of menstrual blood, causing it to pool and oxidize before being passed.

Infections

Certain infections of the reproductive tract, such as pelvic inflammatory disease (PID) or sexually transmitted infections (STIs), can cause abnormal discharge that is dark or black. These infections are often accompanied by other symptoms like a foul-smelling odor, fever, itching, or pain during urination or intercourse. Seeking prompt medical attention for these infections is crucial to prevent long-term complications.

Postpartum Bleeding (Lochia)

For women who have recently given birth, black or dark clots are a normal part of the postpartum healing process, known as lochia. This discharge, which can last for several weeks, consists of blood, mucus, and tissue from the uterus. The color starts out bright red and gradually becomes darker as the flow lessens and the blood oxidizes.

Miscarriage

Dark or black blood clots can, in some instances, be a sign of a miscarriage, particularly an early or 'missed' miscarriage where the embryo has stopped developing but hasn't been expelled from the body. This may or may not be accompanied by other symptoms like cramping or abdominal pain. Any bleeding during pregnancy should be evaluated by a healthcare professional.

When to Consult a Healthcare Provider

While black blood clots are often harmless, there are specific signs that indicate a need for medical evaluation. Monitoring your symptoms and understanding your cycle can help you determine when to seek advice. Seek professional medical advice if you experience any of the following with black blood clots:

A strong, unpleasant, or foul odor.
Severe or unusual pelvic pain and cramping.
Consistent black clots that are larger than a quarter.
Bleeding that lasts longer than your typical cycle.
Fever or other signs of infection.
Bleeding during pregnancy or after menopause.
Increased fatigue, weakness, or dizziness.

Comparison of Normal vs. Concerning Black Clots


Feature	Normal Black Blood Clots	Concerning Black Blood Clots
Timing	Occurs at the start or end of your menstrual cycle.	Appears consistently throughout the cycle or at unexpected times.
Associated Symptoms	Typically has no other symptoms.	Accompanied by foul odor, severe pain, or fever.
Size	Small and infrequent, usually under a quarter.	Large, numerous, or appearing in rapid succession.
Duration	Lasts for only a day or two.	Continues for a prolonged or unusually long time.
Other Factors	Associated with slow, light flow.	May occur alongside heavy, irregular bleeding or be accompanied by weakness.

Steps to Take if You Notice Black Blood Clots

Track Your Cycle: Keep a record of your menstrual cycle, noting the timing, duration, and any accompanying symptoms. This information can be valuable for your healthcare provider.
Stay Hydrated: Proper hydration promotes smoother blood flow and can help minimize clot formation.
Ensure Proper Hygiene: Especially when using products like tampons, be sure to change them regularly to prevent irritation and infection.
Know Your Body's Normal: Recognize what is typical for your body. Changes in cycle patterns, flow, or symptoms can be the most telling indicators of a potential issue.
Seek Professional Guidance: If you are ever in doubt or notice concerning symptoms, consult a healthcare professional for a proper diagnosis. A doctor can perform a physical exam or other tests to determine the underlying cause. An authoritative source for women's health can be found at the American College of Obstetricians and Gynecologists.

Conclusion

While the appearance of black blood clots can be alarming, in most cases, it is simply older, oxidized blood and a normal part of the menstrual cycle, particularly during a slow flow. However, paying attention to the context in which these clots appear is essential. The presence of persistent, large clots, accompanied by a foul odor, fever, or severe pain, warrants medical evaluation to rule out underlying conditions such as fibroids, infections, or hormonal imbalances. Tracking your cycle and seeking professional guidance when unsure can provide reassurance and ensure optimal reproductive health.

Frequently Asked Questions

No, black blood clots are not always serious. They are most often a normal occurrence, particularly at the beginning or end of a menstrual cycle, and are caused by old blood that has oxidized. However, if accompanied by other symptoms, they can indicate an underlying condition.

You should be concerned and see a doctor if you experience black blood clots alongside a foul-smelling odor, severe abdominal pain, a fever, very heavy bleeding, or if they appear outside of your normal period cycle.

Yes, hormonal imbalances, such as those associated with conditions like PCOS or perimenopause, can affect the timing and flow of your period. This can lead to older blood and tissue being shed, resulting in darker, sometimes black, clots.

Normal black clots are small, infrequent, and appear with a light flow at the start or end of your period. Concerning clots are larger than a quarter, appear with heavy or persistent bleeding, and are accompanied by other worrying symptoms like intense pain or a bad odor.

Black spotting can occasionally occur with implantation bleeding in early pregnancy. However, dark blood clots can also be a sign of a miscarriage. Any bleeding during pregnancy should be evaluated by a healthcare professional.

Yes, it is normal to experience dark or black clots after childbirth. This is part of lochia, the normal postpartum bleeding, which initially contains darker, older blood as the uterus sheds its lining.

Yes, a retained foreign object like a forgotten tampon can block the outflow of menstrual blood. This can lead to infection and cause black or foul-smelling discharge. Medical attention should be sought immediately if this is suspected.