Causes of Deep Vein Thrombosis

Clots form when three conditions come together - doctors call them Virchow's triad - and nearly every DVT risk factor feeds one of the three.

The triad behind every clot

• Slow blood flow (stasis): blood pooling in the leg veins - bed rest after surgery or illness, long flights or drives with knees bent for hours, paralysis, a leg in plaster
• Vein wall injury: surgery (especially hip, knee, abdomen), fractures, trauma, previous DVT leaving scarred veins
• Blood that clots too easily (hypercoagulability): cancer and its treatments, pregnancy and the first weeks after delivery, estrogen (combined pills, HRT), inherited clotting disorders (thrombophilia), severe infection, dehydration

The everyday risk list

• Major surgery or hospitalisation - the biggest single trigger
• Immobility: long flights, long drives, desk-bound days - risk rises after about 4 hours of stillness
• Pregnancy and the 6 weeks after delivery
• Cancer, active or under treatment
• Estrogen-containing contraception or hormone therapy - risk multiplies when combined with smoking
• Age over 60, obesity, smoking
• Previous DVT or PE, or close family members with clots - ask about thrombophilia testing if clots run young in the family
• Varicose veins add a modest risk

Provoked vs unprovoked

Doctors sort DVT into provoked (a clear trigger - surgery, flight, pregnancy) and unprovoked (no trigger found). Unprovoked clots prompt a hunt for hidden causes - inherited clotting disorders and, occasionally, an undiagnosed cancer.

One-line summary

DVT happens when slow flow, vein injury and clot-prone blood combine - the classic setups being surgery, bed rest, long travel, pregnancy, cancer and estrogen, with age, obesity, smoking and family history stacking the odds.