Why Is Your Period Blood Black? Causes and Warning Signs

Why Does Period Blood Turn Black?

Menstrual blood does not always have the same color throughout a period.

Blood that leaves the uterus quickly is more likely to appear bright or dark red. When the flow slows, blood may remain in the reproductive tract longer. During this time, oxidation causes it to become darker.

Depending on how long it takes to leave the body, menstrual blood may look:

• Bright red - fresh blood leaving the uterus quickly
• Dark red - blood that has started to darken
• Brown - older, oxidized blood
• Nearly black - older blood that remained in the reproductive tract longer

The color may also look darker when blood mixes with tissue from the uterine lining, cervical mucus and vaginal fluid.

Blood color alone cannot identify a medical condition. Timing, flow, pain, odor and changes from your usual cycle provide more useful information.

Is Black Period Blood Normal?

Black period blood is usually normal when it:

• Appears during the first or final days of a period
• Occurs while the flow is light
• Lasts for a short time
• Does not have a strong or unusual odor
• Is not accompanied by severe pain or fever
• Follows your usual menstrual pattern

It may also appear during an irregular or delayed cycle because the uterine lining does not always shed at the same speed.

Irregular bleeding is more common during adolescence, after changing hormonal contraception and during perimenopause. Conditions that affect ovulation, such as PCOS, can also cause delayed, missed or unpredictable periods.

These conditions do not directly turn blood black. Instead, they can produce lighter or irregular bleeding that takes longer to leave the body.

A sudden change from your usual pattern is more important than a single episode of dark blood. Persistent changes may be part of an abnormal menstruation pattern that deserves medical assessment.

Black Period Blood vs Black Vaginal Discharge

Black period blood and black vaginal discharge can look similar, but the timing and accompanying symptoms help distinguish them.

Normal vaginal discharge is usually clear or white and does not cause a strong odor, pain or burning. Dark discharge with these symptoms is different from seeing a small amount of dark blood during a normal period.

A forgotten tampon or another object left in the vagina can cause dark, foul-smelling discharge. It may also cause itching, irritation, fever or pelvic discomfort. Seek prompt medical care if you think an object may be retained and you cannot remove it easily.

Why Can Black Period Blood Look Thick?

Menstrual blood contains more than blood alone. It also contains tissue from the uterine lining, cervical mucus and vaginal fluid.

This mixture can make the flow look thick, stringy or jelly-like. Dark blood may appear especially thick when the flow is light because there is less liquid moving through the reproductive tract.

Thick black period blood is generally less concerning when:

• It appears only during part of the period
• The amount of bleeding is not unusually heavy
• There is no severe or worsening pain
• There is no strong odor
• Your period follows its normal schedule

Thick blood should be evaluated if it repeatedly occurs with heavy bleeding, large clots, severe pain or bleeding that lasts longer than seven days.

Are Black Period Blood Clots Normal?

Occasional small clots can be a normal part of menstruation, particularly during heavier flow.

Clots form when blood collects briefly before leaving the uterus. They contain blood and tissue from the uterine lining, so they may appear soft, thick or jelly-like. Older clots may look dark red, brown or almost black.

Doctors pay more attention to the size and frequency of clots than to their color.

According to the American College of Obstetricians and Gynecologists, repeated clots about the size of a quarter or larger can be a sign of heavy menstrual bleeding.

Arrange a medical appointment if clots:

• Are repeatedly the size of a quarter or larger
• Appear during most periods
• Occur with severe cramps or pelvic pain
• Accompany bleeding that lasts more than seven days
• Occur with fatigue, weakness or shortness of breath
• Appear with bleeding that disrupts sleep or daily activities

If you are unsure whether your flow is excessive, compare your symptoms with other signs of heavy period bleeding.

Less Common Causes of Black Period Blood

Older blood explains many cases of black period blood. Less commonly, another condition may change the timing or amount of bleeding.

Hormonal Cycle Changes

Hormonal changes can make periods lighter, heavier, delayed or irregular. A small amount of blood may then leave the uterus slowly and appear dark.

Possible contributors include:

• PCOS
• Thyroid disorders
• Perimenopause
• Significant weight changes
• Stress
• Starting, stopping or changing hormonal contraception

Hormonal changes affect the bleeding pattern rather than changing the color directly.

Infection or a Retained Vaginal Object

An infection may cause abnormal bleeding that mixes with vaginal discharge and appears dark. A retained tampon or another vaginal object may produce a similar appearance.

Warning signs include:

• A strong or foul odor
• Yellow, green or gray discharge
• Pelvic pain
• Fever
• Burning during urination
• Pain during sex
• Vaginal itching or irritation

Black blood without these symptoms is less likely to be caused by an infection.

Retained Tissue

After childbirth, miscarriage or certain gynecologic procedures, tissue can occasionally remain inside the uterus.

This may cause persistent dark bleeding or spotting. Pelvic pain, fever, a strong odor or prolonged bleeding increases the need for prompt evaluation.

Fibroids or Adenomyosis

Fibroids and adenomyosis do not directly make menstrual blood black. However, they can cause heavy, painful or prolonged periods. Blood remaining after the heaviest flow slows may appear dark brown or black.

Possible signs include:

• Heavy menstrual bleeding
• Repeated large clots
• Periods lasting longer than seven days
• Pelvic pressure
• Severe menstrual pain

Adenomyosis can resemble other gynecologic conditions, so understanding the differences between adenomyosis and endometriosis may help when discussing symptoms with a healthcare professional.

Cervical Narrowing

A narrowed cervical opening can rarely slow the passage of menstrual blood. This is more likely after cervical surgery, another procedure or scar formation.

Severe narrowing may cause very light bleeding, worsening cramps or cyclic pelvic pain. Black blood without these symptoms is unlikely to be caused by cervical narrowing.

When Should You Worry About Black Period Blood?

Black blood at the beginning or end of an otherwise normal period usually does not require medical treatment.

Arrange a medical appointment if you notice:

• A sudden or persistent change in your usual period
• Black bleeding between periods
• Repeated clots about the size of a quarter or larger
• Bleeding lasting longer than seven days
• Severe or worsening menstrual pain
• A strong or foul odor
• Unusual discharge
• Persistent fatigue, weakness or shortness of breath
• Any bleeding after menopause

Seek urgent medical care if you experience:

• Heavy bleeding that soaks through a pad or tampon every hour for several hours
• Heavy bleeding with dizziness, weakness or fainting
• Severe or sudden pelvic pain
• Fever with pelvic pain or foul-smelling discharge
• Heavy bleeding during pregnancy
• One-sided pain, shoulder pain or fainting when pregnancy is possible
• Heavy bleeding after childbirth, miscarriage or a gynecologic procedure

These symptoms do not confirm a specific condition, but they require timely assessment.

How Do Doctors Check the Cause?

Testing is not usually needed when black blood appears briefly during a normal period without other symptoms.

If the bleeding is new, persistent or accompanied by concerning symptoms, a healthcare professional will first ask about:

• When the dark blood appears
• How long the bleeding lasts
• How heavy the flow is
• The size and frequency of clots
• Pain, fever, odor or unusual discharge
• Changes in cycle length
• Pregnancy possibility
• Contraception use
• Recent childbirth, miscarriage or gynecologic procedures

A pelvic examination may be recommended when infection, a retained object, cervical changes or another source of bleeding is suspected.

If pregnancy is possible, a pregnancy test is often an early step. Blood tests may be used to check for anemia when bleeding is heavy or to investigate other suspected causes.

A pelvic ultrasound may be recommended when symptoms suggest fibroids, adenomyosis, an ovarian condition, retained tissue or another structural cause.

The tests chosen depend on the complete bleeding pattern. The black color itself does not require a special test.