Blood clots during your period: should you worry?

During your period, the lining of your uterus (the endometrium) sheds and flows out, mixed with blood. To keep this mixture fluid, your body makes natural anticoagulants that dissolve small clots as they form ^[1].

When the flow is very heavy, these anticoagulants can't keep up. The blood clots before it leaves and forms those jelly-like clumps ^[1]. Think of a sink: the drain handles a trickle but overflows if you turn the tap on full.

These clots contain blood, fragments of endometrium, and fibrin (a clotting protein) ^[1]. Their dark colour comes from oxidation in the air. It's not "dirty" blood: it's just biology.

They mostly appear on the first one to two days, at peak flow ^[2]. As small as a grape (under a centimetre) or a small coin (one to two centimetres), and occasional, they are part of your body's normal functioning ^[3].

Many women spend years with very heavy periods thinking "that's just how it is." You have every right to question what you're going through.

Three concrete benchmarks deserve your attention:

• clots larger than a two-euro coin (about 2.5 cm), regularly, cycle after cycle: a marker of excessively heavy periods, what doctors call menorrhagia ^[2];
• having to change protection every hour for several hours, or periods lasting more than 7 days: your flow then exceeds the 80 ml per cycle threshold ^[4][5];
• lasting fatigue, breathlessness, pallor, or palpitations: these point to iron-deficiency anaemia ^[6].

This anaemia is common and underdiagnosed: up to 63% of women with menorrhagia develop it without knowing ^[6]. Intense pain that stops your activities, along with large clots, counts too. Your pain is real, and it can be relieved.

Large or recurring clots often have an identifiable cause, and all of them can be treated. Here are the most common.

• Uterine fibroids: non-cancerous growths in the uterine muscle. Up to 70 to 80% of women develop them before age 50, one in three with symptoms ^[7]. Near the cavity, they hamper contractions and increase bleeding ^[7].
• Adenomyosis: lining tissue infiltrates the muscle, which thickens and contracts poorly. It affects around 20% of women and causes heavy bleeding and intense cramps ^[8].
• A hormonal imbalance: excess oestrogen thickens the endometrium, common in cycles without ovulation, in adolescence or perimenopause ^[1][4].
• The copper IUD: 20 to 50% of users report a heavier flow in the first three to six months ^[9]. If it persists, talk to your doctor.

Endometriosis, which affects about 10% of women of reproductive age, can also make periods painful and heavy, especially alongside adenomyosis ^[8].

You don't need to "justify" an appointment: a doubt is enough. To describe your periods well, note the size of your clots, how often you change protection, and your fatigue.

Two tests come up often. A pelvic ultrasound (painless, a few minutes) looks for fibroids or adenomyosis ^[5]. A blood test checks haemoglobin and ferritin, the most reliable marker of your iron stores: below 30 micrograms per litre, they are depleted ^[6].

Several treatments can be discussed with your doctor:

• tranexamic acid, non-hormonal, taken only on heavy days: it cuts flow by 34 to 54% ^[10][5];
• the hormonal IUD (levonorgestrel), a first-line option, which reduces blood loss by 71 to 96% within a year ^[5]; the combined pill lowers it by 40 to 50% ^[4];
• iron supplementation if anaemia is confirmed, oral or intravenous ^[6].

When a fibroid is to blame, removing it through the natural passages (no incision) resolves 80 to 90% of bleeding ^[7]. If a doctor has dismissed your symptoms before, how you feel stays valid: you can insist or seek another opinion.