What can cause black blood during menstruation?

May 7, 2026

If you've ever noticed your period blood looking darker than usual, maybe even black, you're definitely not alone.

While menstrual blood is usually red, it can show up in different shades from month to month, and that's completely normal.

So why does your period blood colour change from red to dark brown or even black? Let’s figure this out.

What causes dark menstrual blood?

During your period, what comes out isn't blood alone. It also includes the shedding of the uterine lining and other vaginal secretions.

The colour change depends on how these components interact and how long they take to exit the body.

Many people notice black blood at the beginning or end of their periods and wonder what causes it. Blood never really turns black; in some cases, it can become increasingly dark and appear black.

Is black period blood normal?

If you are wondering why your period blood is black, the answer isn't always straightforward. Sometimes it’s normal, and sometimes it could be a symptom of something else.

You shouldn't be alarmed if your periods follow a predictable pattern, and black period blood shows up right before or after your period.

During the beginning and the end of your period, the menstrual flow tends to be slower than normal. As the blood takes longer to leave the uterus, it reacts with oxygen molecules on its way.

This is called oxidation and occurs when blood stays in the uterus or vagina for a longer period. The oxidation of period blood causes a chemical reaction that darkens its colour.

There can be different reasons why your flow may be delayed, which can oxidise period blood.

Sometimes, it can be a response to certain hormone imbalances, like stress, thyroid disorders, and contraceptive changes, which can delay menstrual flow. In these cases, the blood also gets more time to darken.

1. Hormonal changes: Dark brown period blood can also be a result of a hormonal imbalance. Your body can naturally experience hormonal shifts, which affect how slowly or quickly your period flows.

2. PCOS: If you have PCOS, this can cause irregular cycles as the uterine lining builds up for a longer time, then sheds as thick, dark material when periods arrive. This may darken the colour of your flow.

3. Contraceptive changes: Birth control methods often slow the flow, especially when you have just started using them or when you are switching devices

Most of the time, black period blood is common. However, in rare cases, this could be a symptom of a larger issue.

When should you worry about dark period blood?

Most of the time, black period blood is nothing to worry about. But there are a few situations where it's worth checking in with your doctor. If you notice black discharge for more than three days, that could be a symptom of something else, and you should consult a doctor.

1) Retained period blood

In rare cases, something called retained menstruation (Hematocolpos) can occur. This is when the menstrual blood cannot leave the vaginal canal.

When the blood isn't able to leave the body, it collects in the vagina, growing darker over time. There can be different reasons why the blood is getting blocked. Certain conditions can partially or completely block the vaginal opening, such as a vaginal septum or imperforate hymen (hymen covering the whole vaginal opening). In rare cases, the absence of a cervix can also cause retained menstruation.

2) Something could be stuck inside the vagina

A foreign object getting stuck in the vagina can also cause black blood or black discharge. It happens more often than you'd think: someone inserts a tampon and genuinely forgets it's there. This can cause black blood.

Other objects that might get lodged in the vagina include condoms, sex toys, and contraceptive devices such as sponges, diaphragms, rings, and cervical caps.

Over time, these can irritate the vaginal lining and lead to an infection. Aside from black period blood, an infection might cause other symptoms, such as:

Vaginal discharge with an unpleasant odor
Discomfort or itching in or around the vagina
Rash or swelling of the genital area
Fever
Difficulty urinating
Pelvic or abdominal pain

If you see black period blood in combination with any of the above symptoms and think something is stuck inside you, seek help immediately. In rare cases, it could lead to a life-threatening infection known as toxic shock syndrome.

3) Sexually Transmitted Infections (STIs)

Sometimes, black blood can be a symptom of an underlying infection. For example, pelvic inflammatory disease can produce black discharge, STIs such as gonorrhoea and chlamydia can do the same.

So if your flow is black and you experience symptoms like a foul smell, vaginal itching, or pelvic pain, that is a sign to check in with a doctor. But black discharge alone doesn't confirm infection.

Read this blog to learn more about STIs and other types of vaginal infections.

4) Uterus health

For the uterus to properly prepare for the menstruation phase, uterine health is important as it determines the flow of your menstruation cycles. If you have developed uterine fibroids, uterine polyps, or endometrial tissue, this could also cause black periods as it disrupts the normal flow.

5) Pregnancy/miscarriage

You might notice dark blood during early pregnancy, in the form of light spotting. This isn’t concerning, especially if it’s the first trimester. According to the Cleveland Clinic, between 15% and 25% of all pregnant women experience bleeding or spotting in the first 12 weeks of pregnancy. However, if you are experiencing bleeding that is increasing, cramps, back pain or passing black clots, you should seek help immediately. In this case, black blood clots can be a sign of miscarriage.If you are pregnant, any kind of bleeding should be communicated to your doctor.

6) Cervix Health

If you notice black blood with irregular bleeding, particularly after sexual activity or during menstruation, it could be an early indication of cervical cancer.

Since cervical cancer doesn't have very noticeable symptoms, it might be a good idea to check for any changes in the cervix with a doctor.

Most instances of black blood are not cancer-related, and fortunately, the chances of this being a reason are low.

How can you maintain healthy menstrual cycles?

Your daily habits shape how your body functions, and you can make lifestyle changes to reduce hormonal fluctuations and support healthy periods. These include:

A balanced diet is often underestimated but can drastically improve your health. Eating food rich in iron and vitamin-C supports menstrual health and may affect flow patterns. Your diet can impact hormone levels and how the uterine lining builds and sheds.

Stress and inadequate sleep can alter cycle regularity, which affects blood appearance. You can create systems that can help your body recover from your daily stresses, like yoga, meditation, and breathing exercises.

Extreme exercise or physical exertion can also dysregulate your menstrual cycle. Take it a little slow, giving your body time to recover and find a balance.

Blood flows more smoothly when you stay hydrated. Dehydration can thicken it and make the colour seem darker.

Your period can tell you a lot about what's going on in your body. While seeing black blood might feel strange at first, it's usually just your body doing its thing.

One way to stay curious about your cycle is by using menstrual cups. It changes the way you see periods because you can see how much flow there is in a day and what the colour looks like during your cycle. And if something feels off, always consult a medical professional to be sure.