What causes blood clots during your period?

Bottom lineSmall period blood clots (under about 2 cm) are normal, especially on heavy days and overnight, forming when blood pools faster than anti-clotting agents can act; see a doctor if clots are larger than about 2.5 cm, come with very heavy bleeding or anemia symptoms, or are a new worsening pattern, since these can signal fibroids, hormonal imbalance, or adenomyosis.

Small period clots are normal, especially on your heaviest days. They form when blood pools faster than your body's anti-clotting agents can keep up.

What's normal

• Small clots (smaller than a 10p coin or about 2 cm), bright or dark red
• More common on heavy-flow days and overnight
• Often darker first thing in the morning

When clots are worth checking

• Large clots (bigger than about 2.5 cm)
• Clots with very heavy bleeding (soaking a pad or tampon every 1 to 2 hours)
• Clots plus anemia symptoms like fatigue or breathlessness
• A new or worsening pattern

Large or frequent clots often go with heavy periods, which can be caused by fibroids, hormonal imbalance, adenomyosis, or PCOS/PMOS.

What to do

Note clot size (compare to a coin) and how often you change protection. Bring that to a clinician if clots are large or come with heavy flow.

Learn more in what is a heavy period, and track clots and flow in Femora.

Femora lets you log clots and flow so patterns and changes are easy to spot.