1. Combined birth control pills clot risk
Combined oral contraceptives contain estrogen and progestin, and it is the estrogen component that increases the risk of venous thromboembolism (VTE), which includes deep vein thrombosis (DVT) and pulmonary embolism (PE). This risk is real but often overstated when presented as a relative increase without context.
In absolute terms, among 10,000 women per year:
- Those not pregnant and not on the pill have approximately 1 to 5 cases of VTE.
- Women taking combined pills experience about 3 to 9 cases.
- Pregnant women have about 5 to 20 cases.
- Women in the 12 weeks postpartum have the highest risk, roughly 40 to 65 cases.
This means pregnancy and the postpartum period carry a higher clot risk than the pill itself. For women using combined pills to prevent pregnancy, the comparison should be between pill use and pregnancy, not pill use versus doing nothing. The pill often results in a net lower clot risk when considering this.
However, certain factors significantly increase clot risk on the pill, including smoking (especially over age 35), a personal or family history of clots, and inherited clotting disorders. Women with these risk factors should discuss alternatives with their healthcare provider.
2. Hormone replacement therapy clot risk
Hormone replacement therapy (HRT) used for menopausal symptoms can also increase the risk of venous clots, but the route of administration dramatically affects this risk.
Oral estrogen is absorbed through the digestive system and passes through the liver first. This “first-pass” effect causes the liver to increase production of clotting factors, raising the risk of venous thrombosis.
Conversely, transdermal estrogen—delivered through patches or gels—enters the bloodstream directly, bypassing the liver and thus not triggering increased clotting factor production. Clinical guidelines recognize that transdermal estrogen does not carry the same clot risk as oral estrogen.
For women concerned about clot risk but needing menopausal hormone therapy, discussing transdermal options with their prescriber can significantly reduce risk while managing symptoms effectively.
