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Menstrual blood problems: Clots, colour and thickness

It is normal for the colour and consistency of menstrual blood to vary during a period.

However, some unusual changes in colour, thickness or clotting may indicate a blood problem that may need medical attention.

Seek medical advice if you have concerns.

What's happening during a woman's period, and how long does a normal period last?

During your menstrual cycle, the lining of your uterus thickens to get ready for pregnancy. Then, during your period, your body sheds the uterus lining along with blood. The amount of blood and fluid lost is usually between four and 12 teaspoons each cycle.

The average menstrual cycle lasts 28 days. 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 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 colour. Often, these clots are released on the heaviest days of bleeding. The presence of multiple clots in your flow may make your menstrual blood seem thicker 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 10p piece, you should seek medical advice to rule out any conditions that might be causing an abnormal period.

Are darker colours and thicker flows normal in menstrual blood?

Sometimes 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 colour change. It happens when the blood is older and not being expelled from the body quickly.

Temporary thick heavy flow isn't necessarily a cause for concern. However, regular heavy periods justify a trip to the doctor to check your blood count. Many women become accustomed to heavy periods, considering them to be normal. Over time, however, the excess monthly blood loss leads to anaemia (an abnormally low level of oxygen-carrying blood cells), potentially causing weakness or fatigue. If you ever feel something's not right with your period, seek medical advice.

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