Birth Control Pills and Blood Clots: What the Progestin Generation Actually Changes

Almost every article about the pill and blood clots stops at "there is a small increased risk." That is true and useless. The number that matters is how small, compared to what, and whether the specific tablet in your hand sits at the low end of the range or the high end. The prescribing information for combined oral contraceptives answers all three questions, and the answers are more specific than most summaries admit. This guide works through the labelled figures, the difference between progestin generations, and the situations where the arithmetic changes sharply.

The absolute numbers, not the relative ones

Relative risk is the figure that gets quoted because it sounds dramatic. Absolute risk is the figure that tells you something. United States labelling for combined oral contraceptives presents venous thromboembolism incidence across four groups, expressed per 10,000 woman-years: roughly 1 to 5 events in women who are neither pregnant nor using a combined pill, roughly 3 to 9 in combined pill users, roughly 5 to 20 during pregnancy, and roughly 40 to 65 in the twelve weeks after delivery.

Read that sequence again. The postpartum period carries something in the order of ten times the clot risk of pill use. Pregnancy itself carries more than the pill. This is the context that gets stripped out when the pill is discussed in isolation, and it is the reason regulators keep these products on the market rather than withdrawing them.

Why the estrogen is the part that clots

The progestin gets the attention in comparison studies, but the estrogen component is doing most of the mechanical work. Ethinyl estradiol passes through the liver and drives production of estrogen-sensitive hepatic globulins and coagulation factors. A 2023 comparative analysis of the estrogens used in contraception (PMID 37863464) describes this potency directly: the marked rise in hepatic globulins and clotting factors is the notable feature of ethinyl estradiol relative to natural estradiol and to estetrol, and the natural estrogens appear less stimulating to coagulant proteins.

That mechanism explains why estrogen dose reduction has been the main safety strategy for fifty years. Formulations moved from 50 micrograms of ethinyl estradiol down to 30 and then 20. Among the products we stock, Duoluton L sits at 0.05 mg and Ovral G at 0.05 mg, while Loette and Femilon sit at 0.02 mg. The lower figure is not a marketing distinction. It is the single variable with the longest track record of changing outcomes.

What the label says about relative risk

Combined oral contraceptive labelling reports case control study findings as a relative risk of about 3 for a first episode of superficial venous thromboembolic disease, 4 to 11 for deep vein thrombosis or pulmonary embolism, and 1.5 to 6 in women who already have predisposing conditions. Cohort studies produce lower figures, around 3 for new cases and around 4.5 for cases requiring hospital admission.

The spread in those ranges is wide because the underlying studies used different populations, different formulations and different diagnostic thresholds across several decades. A range of 4 to 11 is not precision. It is an honest statement that the effect is real and its size is uncertain.

Second generation versus third generation progestins

Progestins are grouped by when they were introduced. Levonorgestrel and norgestrel are second generation. Desogestrel and gestodene are third generation. The label for desogestrel-containing products states the comparison plainly: several epidemiologic studies indicate that third generation products, including those containing desogestrel, carry a higher venous thromboembolism risk than certain second generation products, with an approximate two-fold increase corresponding to an additional 1 to 2 cases per 10,000 women-years. It also notes that other studies have not reproduced that two-fold figure.

One to two additional cases per 10,000 women-years is the concrete translation of "twice the risk." For most healthy women it is a small number. For a woman with a family history of unprovoked clot in a first degree relative, it stacks on top of a baseline that is already elevated, and the choice between a levonorgestrel product such as Ovral L and a desogestrel product such as Novelon becomes a real clinical decision rather than a preference.

Where drospirenone sits

Drospirenone is sometimes called fourth generation. It is a spironolactone analogue, which is why it has mild antimineralocorticoid and antiandrogenic activity and why it behaves differently on fluid retention and skin. Studies comparing drospirenone products against levonorgestrel products for clot risk have not agreed. Some reported roughly a three-fold difference. Others found no difference at all. Regulators reviewed the evidence and left the products on the market with the disagreement described in the labelling rather than resolved.

Our drospirenone range includes Yasmin and Yaz from Bayer Zydus, and the Sun Pharma and Cipla equivalents Dronis 30, Dronis 20, Crisanta and Crisanta LS. The dose difference between the 0.03 mg and 0.02 mg estrogen versions is covered in detail in our Yaz versus Yasmin comparison.

Risk does not accumulate with years of use

This is the single most commonly misunderstood point, and the label is unambiguous about it. Thromboembolic risk from oral contraceptives is not related to length of use, and it disappears after the pill is stopped. Ten years of continuous use does not build a debt that comes due later.

What does change is that risk is highest in the first months of use and after any restart following a break of a month or more. Stopping for three months and starting again resets you into that higher early window. Continuous use, counterintuitively, avoids repeating the riskiest phase.

The smoking threshold at 35

Every combined oral contraceptive sold in the United States carries the same boxed warning at the top of its label. Cigarette smoking increases the risk of serious cardiovascular events from combined oral contraceptive use, the risk rises with age and with the number of cigarettes, and combined products are contraindicated in women over 35 who smoke.

Contraindicated is a stronger word than "discouraged." Separately, the label estimates the relative risk of myocardial infarction in current oral contraceptive users at two to six, concentrated almost entirely in smokers and in women with hypertension, high cholesterol, severe obesity or diabetes. Under 30 the risk is described as very low. For a woman over 35 who smokes, a progestin-only option such as Cerazette or Dronis P, or an intrauterine device such as Mirena, removes the estrogen from the equation entirely.

Surgery and prolonged immobilisation

The label reports a two to four-fold increase in relative risk of post-operative thromboembolic complications with oral contraceptive use, and gives specific timing. Where feasible, oral contraceptives should be stopped at least four weeks before elective surgery of a type associated with increased thromboembolism risk, and for two weeks afterwards, and during and after prolonged immobilisation.

Four weeks before, two weeks after. That timing gets missed constantly because the surgical team asks about anticoagulants and the patient does not think of a contraceptive pill as a blood medicine. Tell the surgical team. Arrange alternative contraception for those six weeks.

The postpartum window

Because the period immediately after delivery already carries the highest clot risk of any state in the table above, combined oral contraceptives should be started no earlier than four weeks after delivery in women who choose not to breastfeed. Starting earlier layers pill risk on top of a baseline that is already at its peak.

Clotting disorders that nobody screens for

Factor V Leiden, prothrombin gene mutation, protein C or S deficiency and antithrombin deficiency all raise baseline clot risk, and none of them is routinely tested before a contraceptive prescription. Population screening is not recommended because the tests are expensive and the conditions are uncommon enough that mass testing would generate more harm through false reassurance and anxiety than it prevents.

What replaces screening is a family history question. An unprovoked deep vein thrombosis or pulmonary embolism in a parent or sibling, particularly before age 50, is the signal that should prompt a conversation about testing or about choosing a method without estrogen. The label lists inherited or acquired hypercoagulopathies as an outright contraindication.

Migraine with aura

Migraine headaches with focal neurological symptoms, and migraine with or without aura in women over 35, appear in the contraindication list for combined products. The concern here is ischaemic stroke rather than venous clot, and it is a genuinely different mechanism from the venous risk discussed above.

Aura means visual, sensory or speech disturbance preceding the headache: zigzag lines, a blind spot that expands, numbness spreading up an arm. A severe headache with light sensitivity and nausea but no preceding neurological symptom is migraine without aura, which under 35 is not an automatic contraindication. The distinction is worth getting right before assuming you cannot use a combined pill.

The Hepatitis C interaction that is easy to miss

During clinical trials of the hepatitis C combination regimen containing ombitasvir, paritaprevir and ritonavir, with or without dasabuvir, alanine aminotransferase elevations above five times the upper limit of normal, including some above twenty times, were significantly more frequent in women taking ethinyl estradiol-containing medicines. The labelled instruction is to stop the combined contraceptive before starting that regimen and restart approximately two weeks after it finishes.

What a clot actually presents as

Deep vein thrombosis usually affects one leg: calf or thigh pain that does not match any injury, swelling on one side only, warmth, and sometimes a colour change. Symmetrical swelling in both legs is far more often something else.

Pulmonary embolism presents as sudden breathlessness, chest pain that worsens on breathing in, coughing, sometimes coughing blood, and a fast heart rate. Retinal thrombosis has been reported in case series, and the label directs that oral contraceptives be discontinued for unexplained partial or complete vision loss, new bulging of the eye, double vision, swelling of the optic disc, or retinal vascular changes. Any of these needs emergency assessment on the same day, not a routine appointment.

Lowering risk without abandoning contraception

Choosing the lowest effective estrogen dose is the intervention with the strongest evidence behind it. Choosing a second generation progestin such as levonorgestrel, available in Ovral L, Loette, Triquilar and Unwanted 21 Days, sits at the low end of the comparative range. Removing estrogen entirely through a progestin-only pill or an intrauterine system removes the estrogen-driven component of the risk.

Practical measures matter too. Move and hydrate on flights over four hours. Stop smoking, which is the single largest modifiable multiplier in the whole picture. Get blood pressure checked, since uncontrolled hypertension is a listed contraindication rather than a minor consideration. And know what you are taking, because "the pill" covers formulations whose estrogen doses differ by a factor of two and a half.

How our range differs on the variables that matter

Estrogen dose is the first variable. Femilon, Loette and Yaz use 0.02 mg of ethinyl estradiol. Novelon, Ovral L, Femovan, Yasmin and Dronis 30 use 0.03 mg. Duoluton L and Ovral G use the older 0.05 mg.

Progestin generation is the second. Levonorgestrel and norgestrel products are second generation. Femilon, Novelon and Femovan are third generation. The drospirenone products sit in the disputed group. Diane 35 and Ginette 35 contain cyproterone acetate and are licensed for androgen-related conditions rather than as first-line contraception in most markets, which is a separate discussion from the one above.

Safety information

Combined oral contraceptives are prescription only medicines in the United States, United Kingdom, Australia and Canada. Do not use a combined product if you are over 35 and smoke, have had a deep vein thrombosis or pulmonary embolism, have a known clotting disorder, have uncontrolled high blood pressure, have migraine with aura, or have current or previous breast cancer. Seek emergency care for one-sided leg pain or swelling, sudden breathlessness, chest pain, coughing blood, sudden severe headache, weakness on one side, or sudden vision change. Tell any surgeon or dentist planning a procedure that you take a contraceptive pill. Speak to a licensed clinician before starting, stopping or switching any contraceptive. Related reading: our missed pill guide and our metformin for PCOS guide.

References

• FDA prescribing information for combined oral contraceptives, warnings section covering thromboembolic and thrombotic disease, relative risk figures from case control and cohort studies, and the second versus third generation progestin comparison
• FDA boxed warning text common to combined oral contraceptive labelling: cigarette smoking and serious cardiovascular events
• DailyMed label, desogestrel and ethinyl estradiol tablets: warnings section, surgery and immobilisation timing, postpartum initiation, hepatitis C regimen interaction, retinal thrombosis
• FDA prescribing information for Yaz (drospirenone and ethinyl estradiol): contraindications section 4 and warnings and precautions section 5.1
• PubMed PMID 37863464, comparison of estrogenic components used for hormonal contraception, on ethinyl estradiol potency and hepatic coagulation factor stimulation