Reading Contraception Risk: Absolute Versus Relative Numbers

A newspaper can report that a contraceptive "triples" the risk of a blood clot, while a patient leaflet reports that the same method raises the yearly chance of a clot from roughly 3 to roughly 9 in 10,000 users. Both statements can describe one study, and both can be accurate. The first is a relative number, which tells you how much an effect multiplies a baseline; the second is an absolute number, which tells you how many people out of a defined group are actually affected. A relative figure printed without its baseline is a volume knob with no reference point, and it is the format most likely to frighten.

This piece uses two well-documented examples behind the CDC contraception guidance, venous thromboembolism (VTE) and breast cancer, to show how identical data reads as alarming or reassuring depending on which number reaches the reader. This is educational writing, not medical advice.

The clot example

Combined hormonal contraceptives raise the risk of venous thromboembolism. That much is not in dispute. Meta-analyses summarized in the American Society for Reproductive Medicine guideline put the increase at roughly a threefold to fourfold multiple of baseline, with relative risks near 2.8 for levonorgestrel formulations and 3.2 for norethindrone formulations compared with non-use. "Triple the risk" is a defensible headline.

Now the baseline. Among healthy nonpregnant women of reproductive age who do not use these methods, VTE occurs at about 1 to 5 cases per 10,000 woman-years. Users sit at roughly 3 to 15 per 10,000 woman-years. So the tripling moves a small number to a still small number. The absolute change is a handful of extra events per 10,000 women each year, not a coin flip.

The comparison that reframes the entire conversation is pregnancy itself. The same ASRM guideline places VTE risk at 5 to 20 per 10,000 woman-years during pregnancy and 40 to 65 per 10,000 in the postpartum weeks. Pregnancy, the condition contraception prevents, carries a clot risk several times higher than any combined pill does. That single comparison is why the CDC advises against combined hormonal methods in the first 21 days after delivery, when clotting risk peaks, and relaxes the restriction between 21 and 42 days for women without added risk factors. The guidance is not treating the pill as uniquely dangerous. It is ranking risks against one another.

The breast cancer example

Breast cancer follows the same pattern. A 2023 UK nested case-control study and meta-analysis by Fitzpatrick and colleagues in PLOS Medicine found that current or recent use of hormonal contraceptives, combined or progestogen-only, was associated with an odds ratio around 1.2 to 1.3 for breast cancer. In relative terms that is a 20 to 30 percent increase, and a headline can honestly say the pill "raises breast cancer risk."

The absolute picture is where proportion returns. The same authors estimated that five years of use adds roughly 8 extra breast cancers per 100,000 users for those starting between ages 16 and 20, rising to about 265 per 100,000 for those using it between ages 35 and 39, calculated across a 15-year window. The relative increase is nearly constant across ages. The absolute excess is not, because the background risk of breast cancer climbs steeply with age. A 20 percent increase on a tiny baseline stays tiny. The same 20 percent applied to a larger baseline is larger. A relative risk quoted alone erases that distinction, which is why the same percentage can be trivial for a 19-year-old and meaningful for a 38-year-old.

Why the framing matters

The CDC medical eligibility framework, which sorts conditions into four categories running from no restriction to unacceptable risk, is built on exactly this logic of weighing absolute risks against each other and against the risk of the outcome being avoided. It does not ask merely whether a method changes risk. Almost everything changes risk. It asks by how much, from what starting point, and against what alternative.

Three questions convert a frightening relative number into something you can judge:

• Relative to what baseline? A percentage increase means nothing until you know the starting rate.
• How large is the absolute change? Extra cases per 10,000 or per 100,000 is the honest unit, not a multiplier.
• Compared with the alternative? The risk of the option being declined, here pregnancy, belongs in the same frame.

None of this argues that a given method is right for a given person. That decision depends on individual history and belongs with a clinician. The point here is narrower and it concerns literacy. A number that multiplies a rare event is still describing a rare event. When a striking relative figure appears, ask for the two absolute rates behind it, and put the alternative on the same page. The evidence behind contraception risk tends to reassure precisely because it survives that translation.