Mechanisms of Action of Hormonal Emergency Contraceptives
Bibliographic record
Abstract
Hormonal emergency contraceptives have been used to prevent unwanted pregnancy for more than 3 decades. The mechanisms of action of the regimen containing a combination of estrogen and progestin, known as the Yuzpe regimen, and those of the levonorgestrel regimen continue to be controversial, especially over the possibility that these regimens might act by interfering with implantation of the fertilized ovum. We performed a search of the PubMed (1949-July 2009) and EMBASE (1980-July 2009) databases to identify literature on the mechanisms of action of these contraceptive regimens, and data were extracted from pertinent English-language studies. We classified studies according to the approach taken by the investigators to study the actions of emergency contraceptives on pregnancy: an indirect method that uses statistical models to determine whether emergency contraceptives would be as effective as reported if they act only by disrupting ovulation; direct observation of the effects of emergency contraceptives on surrogate outcomes, including ovulation, sperm activity, hormonal levels, and endometrial receptivity to implantation; and analysis of directly observed pregnancy outcomes against statistical data. Acceptability of emergency contraceptives by women and clinicians may depend on personal opinions about when life or pregnancy begins. The evidence strongly supports disruption of ovulation as a mechanism of action. The data suggest that emergency contraceptives are unlikely to act by interfering with implantation, although the possibility has not been completely excluded. The data also suggest that emergency contraceptives are ineffective after ovulation. Women and clinicians who consider implantation or later events to be the beginning of pregnancy should be aware that emergency contraceptives are likely nonabortive by this definition of pregnancy.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.005 | 0.016 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.002 | 0.004 |
| Bibliometrics | 0.007 | 0.004 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.003 | 0.003 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.002 | 0.001 |
| Insufficient payload (model declined to judge) | 0.012 | 0.002 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".