American journal of obstetrics and gynecology
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Am. J. Obstet. Gynecol. · Dec 1991
Further analysis of contraceptive failure of the ovulation method.
Reanalysis of data on the ovulation method of natural family planning collected by the World Health Organization yields the following conclusions. The method is effective during perfect (correct and consistent) use, with a first-year probability of failure of 3.4%. ⋯ Nevertheless, the 13% of cycles characterized by imperfect use had a tremendous impact on the overall failure rate. During the first year of typical use 22.5% of the women in the clinical trial became accidentally pregnant.