Mikolajczyk, R. T., & Stanford, J. B. (2004). S28.3: Validity of methods used to estimate the effectiveness of Emergency Contraception. Biometrical Journal, 46(S1), 61-61. https://doi.org/10.1002/bimj.200490266
Mikolajczyk RT, Stanford JB. S28.3: Validity of methods used to estimate the effectiveness of Emergency Contraception. Biometrical J. 2004;46(S1):61-61. doi:10.1002/bimj.200490266
Mikolajczyk, Rafael T., and J. B. Stanford. "S28.3: Validity of methods used to estimate the effectiveness of Emergency Contraception." Biometrical Journal, vol. 46, no. S1, 2004, pp. 61-61.
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Common methods overestimate emergency contraception effectiveness
This study tested common ways to rate how well emergency contraception works, using cycle data from natural family planning users. The methods overestimated effectiveness by 10 to 15 percentage points when sex was timed near the fertile window. A corrected estimate, which allows for doubt about when that window falls, was 50 to 60 percent. Earlier claims were higher.
Key Findings
Earlier reports had claimed emergency contraception effectiveness above 75%. The paper says the validity of the methods used to estimate effectiveness has not been formally studied.
Using only the cycle just before the current one predicted its length, for estimating the fertile window, as well as averaging five past cycles did.
When sex was timed near the predicted fertile window, standard methods overestimated effectiveness by 10 to 15 percentage points.
Overestimating the daily chance of conception added a further 10 to 20 percentage points of error.
Once the researchers corrected for uncertainty about the fertile window, the pooled effectiveness estimate across trials was 50 to 60 percent. Results did not vary between trials.
Interpretation
This is a methods study. The study is not a trial of real patients using emergency contraception. The design is a simulation. Researchers used cycle data from natural family planning users, whose fertile window is already tracked. They tested if standard math still works when sex happens at different times in the cycle. The results are about the math. They do not describe how well emergency contraception works for any one person. The paper was published as a short conference abstract, so methods detail is limited.
RRM Context
This study depended on natural family planning users tracking ovulation every cycle. The tracking gives exact data. Other methods must guess at it. Restorative reproductive medicine uses that same kind of data. The approach relies on a couple's own charted cycle. A population average is not part of that data. The paper is one example of charted cycle data used as a scientific resource. Charting is more than a personal tracking tool.
Our editorial summary of this paper, not the article's abstract.
