1. Combined contraceptive pills and clot risks
Combined oral contraceptives contain estrogen and a progestin, and their clotting risk is primarily driven by the estrogen component. Estrogen increases the liver’s production of clotting factors, which can raise the risk of venous thromboembolism (VTE), including deep vein thrombosis (DVT) and pulmonary embolism (PE).
It’s critical to understand the actual numbers. For women not pregnant and not on hormonal contraception, the baseline risk of VTE is roughly 1 to 5 cases per 10,000 women per year. Combined pills raise this risk to about 3 to 9 cases per 10,000 women per year. In comparison, pregnancy itself carries a higher risk, 5 to 20 per 10,000, and the first 12 weeks postpartum is even higher, between 40 and 65 per 10,000.
This comparison is often overlooked but essential: taking the pill to prevent pregnancy generally reduces clot risk compared to the alternative state of pregnancy. However, this balance shifts if additional risk factors exist, such as smoking (especially over age 35), a personal history of clots, or inherited clotting disorders. In these cases, the combined pill’s relative risk increase can become clinically significant and warrants discussion with a healthcare provider.
2. Hormone replacement therapy routes
Hormone replacement therapy (HRT) used to manage menopausal symptoms can also affect clot risk, but the route of estrogen administration makes a substantial difference.
Oral estrogen pills are absorbed through the gastrointestinal tract and undergo first-pass metabolism in the liver, which stimulates increased production of clotting factors and thereby raises the risk of venous clots. This risk is measurable and has led to caution around oral HRT in women with clotting risk factors.
In contrast, transdermal estrogen—delivered via patches or gels—bypasses the liver initially, entering directly into the bloodstream. This route does not trigger the liver to increase clotting factors, and current clinical guidelines recognize transdermal estrogen as having no significant increased risk of venous thromboembolism compared to placebo. For women concerned about clot risk but needing effective menopausal symptom relief, switching to transdermal therapy can be a safe alternative.
