Blood clots can form within the first few months of starting hormonal birth control, with the risk peaking during the first six months to one year of use. The overall risk is still low in absolute terms, but understanding the timeline, the type of contraception involved, and your personal risk factors makes a real difference in knowing what to watch for.
The Highest Risk Window
The clot risk from hormonal birth control is not evenly spread across the years you take it. New users face the steepest risk during the first year, and it gradually decreases the longer they stay on the same prescription. For women who carry an inherited clotting disorder (thrombophilia), the numbers are dramatic: the risk of a deep vein clot during the first six months of oral contraceptive use is roughly 19 times higher than during prolonged use, and about 11 times higher during the first full year.
Even without a clotting disorder, the pattern holds. Your body’s clotting system reacts to the introduction of estrogen, and it takes time for that response to settle. This is why restarting the pill after a break resets the clock. If you stop for several months and then begin again, you re-enter that higher-risk early window as if you were a new user.
How Big Is the Risk, Really?
Numbers help put this in perspective. A woman of reproductive age who does not use hormonal contraception has a baseline clot risk of roughly 2 to 4 events per 10,000 women per year. Combined birth control pills containing a second-generation progestin (like levonorgestrel) raise that to about 5 to 8 per 10,000. Pills with newer progestins (desogestrel, gestodene, or drospirenone) push the range to roughly 7 to 12 per 10,000.
That means even in the highest-risk pill category, more than 99.8% of women will not develop a clot in a given year. The risk is real, but it is small. For context, pregnancy itself carries a clot risk several times higher than any oral contraceptive.
Which Types of Birth Control Raise the Risk
Estrogen is the main driver. Combined hormonal contraceptives, meaning any method that pairs estrogen with a progestin (the combination pill, the patch, the vaginal ring), are the ones associated with increased clotting. The type of progestin also matters. Pills with levonorgestrel roughly triple the baseline risk, while pills with drospirenone or desogestrel roughly double that again, reaching six to seven times the baseline.
Progestin-only methods tell a very different story. Multiple large studies have found no statistically significant increase in clot risk with progestin-only pills, hormonal IUDs, or implants compared to non-users. A CDC systematic review confirmed this pattern across several study designs. That’s why progestin-only options are generally considered appropriate even for women with a history of blood clots, where combined pills would be off the table.
Factors That Compound the Risk
Certain personal factors layer on top of hormonal contraception to raise clot risk further. The most significant ones are smoking, being over 35, carrying excess weight, and having a family or personal history of blood clots.
Smoking while on estrogen-containing birth control is especially concerning. It increases the risk of blood clots, strokes, and heart attacks, and the danger rises sharply after age 35. High blood pressure and a family history of heart disease add to this. Obesity also independently raises the likelihood of clotting while on combined contraceptives. These factors don’t just add up; in many cases, they multiply each other. A woman over 35 who smokes and takes a combined pill faces a meaningfully different risk profile than a healthy 25-year-old on the same prescription.
Inherited clotting disorders, such as Factor V Leiden, are the most potent amplifier. Many women don’t know they carry one, since these conditions often cause no symptoms until a clot happens. Routine screening before prescribing birth control is not standard practice, but if you have a strong family history of clots (especially in relatives under 50), it’s worth discussing genetic testing.
How Quickly Risk Drops After Stopping
The good news is that the elevated clotting risk does not linger. Research presented through the American Society of Hematology found that about 80% of the total drop in clotting markers occurs within two weeks of stopping hormonal contraceptives, and 85% within four weeks. In practical terms, your clot risk returns to nearly normal levels within a few weeks of discontinuation. This is why women are typically asked to stop hormonal birth control two to four weeks before a planned surgery or any other situation where immobility raises clot risk.
Symptoms to Be Aware Of
A deep vein thrombosis (DVT) most commonly forms in the leg, and the symptoms tend to appear on one side only. Watch for swelling in one leg or arm that comes on suddenly, pain or tenderness that worsens when you stand or walk, warmth in the swollen area, skin that looks red or discolored, and veins near the skin’s surface that appear larger than usual. Less commonly, clots can form in the abdominal veins (causing deep belly or flank pain) or in the brain’s veins (causing a sudden severe headache, vision changes, or stroke-like symptoms).
The most dangerous complication is a pulmonary embolism, which happens when a clot breaks loose and travels to the lungs. Symptoms include sudden chest pain, shortness of breath, coughing up blood, lightheadedness, and fainting. These symptoms warrant emergency care regardless of whether you’re on birth control, but being aware of the connection can help you and your medical team respond faster during those critical early months of a new prescription.