What Are Period Blood Clots?

Small blood clots during menstruation are normal and common. Clots form when menstrual blood pools in the uterus faster than the body's natural anticoagulants can break it down.

Clots smaller than a 50 kobo coin or a 5-shilling coin, appearing on heavier flow days, require no investigation. Clots larger than this, or clotting accompanied by very heavy bleeding, fatigue, or pelvic pain, warrant medical assessment.

Key Things to Know About Period Clots

• Small clots on heavy flow days are a normal part of menstruation

• Clots form when blood pools and coagulates faster than anticoagulant proteins can break it down

• Clot size is the most reliable indicator of whether something needs attention — the coin rule applies

• Large or frequent clots may indicate fibroids, adenomyosis, or a bleeding disorder

• Heavy menstrual bleeding affects approximately 1 in 3 women at some point in their reproductive life (WHO)

• Iron deficiency anaemia is a common consequence of consistently heavy, clot-heavy periods

Why Do Blood Clots Form During a Period?

During menstruation, the uterine lining sheds and exits through the cervix. The body produces anticoagulant proteins to keep blood flowing. On lighter flow days, these anticoagulants have time to work effectively. On heavier days — typically days one to three — blood can accumulate in the uterus faster than anticoagulants can process it, forming clots before it exits.

This is a mechanical process, not a sign of a clotting disorder. Most period clots are dark red or maroon rather than bright red, because the blood has spent more time in the uterus before passing.

What Size Period Clot Is Normal?

What Causes Abnormally Large or Frequent Clots?

Fibroids

Uterine fibroids are non-cancerous growths in or around the uterine wall. They are the most common cause of heavy menstrual bleeding and large clots in women of reproductive age — affecting an estimated 70% of women by age 50, though many remain asymptomatic (NIH). Fibroids increase the surface area of the uterine lining, producing more blood and larger clots.

Adenomyosis

Adenomyosis occurs when uterine lining tissue grows into the muscular wall of the uterus. It causes heavy, clot-heavy periods and significant cramping. It is frequently underdiagnosed — average time to diagnosis is seven to nine years from symptom onset.

Hormonal Imbalance

Oestrogen dominance — where oestrogen levels are elevated relative to progesterone — causes the uterine lining to thicken more than usual. A thicker lining produces heavier bleeding and more clots when it sheds. This pattern is common in PCOS, perimenopause, and in the cycles following hormonal contraception.

Bleeding Disorders

Von Willebrand disease — the most common inherited bleeding disorder, affecting approximately 1% of the population — reduces the blood's ability to clot normally, paradoxically leading to heavier menstrual flow and larger clots. It is significantly underdiagnosed in women because heavy periods are often dismissed as normal.

Miscarriage

A single episode of unusually large clots, particularly with tissue, outside of a typical period pattern — especially following a late or missed period — can indicate early pregnancy loss. If there is any possibility of pregnancy, a doctor should be seen promptly.

IUD (Non-Hormonal)

A copper IUD increases menstrual blood flow in many women, particularly in the first three to six months after insertion. Larger clots are a common side effect during this adjustment period.

When Are Period Clots a Sign of Anaemia?

Consistently heavy periods with large clots deplete iron stores over time. Iron deficiency anaemia develops gradually and its symptoms are often attributed to other causes:

• Persistent fatigue that does not resolve with rest

• Breathlessness during normal activity

• Heart palpitations

• Pale skin, pale inner eyelids

• Difficulty concentrating

If you experience heavy, clot-heavy periods alongside any of these symptoms, ask your doctor to check your haemoglobin and ferritin levels at your next appointment.

When to See a Doctor

At your next appointment:

• Clots consistently larger than a coin over two or more cycles

• Periods lasting longer than seven days

• Needing to change protection more frequently than every two hours on your heaviest day

• Fatigue, breathlessness, or dizziness during your period

Same day:

• Soaking through a pad or tampon every hour for two or more consecutive hours

• Large clots accompanied by severe pelvic pain or fever

• Clots with tissue, particularly if pregnancy is possible

At the appointment: "I have been passing clots larger than a coin for X cycles. My flow is heaviest on days X and Y. I would like to check my haemoglobin and investigate possible causes including fibroids or adenomyosis."

Medical treatment depends on the underlying cause. Common options include:

• Hormonal contraception — the combined pill, hormonal IUD (Mirena), or injection reduce endometrial thickness and significantly decrease blood loss

• Tranexamic acid — a non-hormonal medication taken during menstruation that reduces blood loss by up to 50% (NICE guidelines)

• Iron supplementation — where anaemia is confirmed

• Surgical options — endometrial ablation or myomectomy (fibroid removal) where fibroids or adenomyosis are the cause

Tracking your flow — number of pads or tampons used per day, clot size, and cycle length — before your appointment gives your doctor the clearest picture and speeds up diagnosis.