Trussell J, Grummer-Strawn L
Office of Population Research, Princeton University.
Fam Plann Perspect. 1990 Mar-Apr;22(2):65-75.
Previously published estimates of probabilities of method and user failure for all contraceptive methods suffer from a serious methodological error and are biased downward, with the extent of bias unknown. Data from a World Health Organization clinical trial of the ovulation method of periodic abstinence were used to provide the first correctly calculated measures of method and user efficacy and to determine the characteristics that distinguish women who consciously take risks from those who do not. Probabilities of pregnancy during the first year are 3.1 percent during perfect use (method failure) and 86.4 percent during imperfect use (user failure). Thus, if used perfectly, the ovulation method is very effective. However, it is extremely unforgiving of imperfect use. Because perfect compliance is difficult for many couples who desire intercourse when it is forbidden by ovulation method rules, and because the risk of pregnancy during imperfect use is so great, the ovulation method cannot be considered an ideal contraceptive method for the typical couple, who are likely to be less compliant than couples who volunteer for a clinical trial. The probability of an accidental pregnancy is greatest when any of the three most serious rules--no intercourse during mucus days, within three days after the day of peak fecundity or during times of stress--are broken. Those who have a poor attitude toward the rules are more likely to take risks, including serious risks. Those who get away with taking a risk (i.e., do not get pregnant) are very likely to take risks again. Because breaking the most serious rules entails a 28 percent risk of pregnancy per cycle, those likely to take risks should be counseled about the probable consequences.
此前发表的关于所有避孕方法的方法失败率和使用者失败率的估计存在严重的方法学错误，且被低估，偏差程度未知。世界卫生组织一项关于周期性禁欲排卵法的临床试验数据被用于首次正确计算方法有效性和使用者有效性的指标，并确定区分有意识冒险的女性和不冒险的女性的特征。在完美使用（方法失败）的情况下，第一年的怀孕概率为3.1%，在不完美使用（使用者失败）的情况下为86.4%。因此，如果完美使用，排卵法非常有效。然而，它对不完美使用极为苛刻。由于对于许多在排卵法规则禁止性交时想要进行性交的夫妇来说，完美遵守规则很困难，而且不完美使用时怀孕风险极大，对于典型夫妇而言，排卵法不能被视为理想的避孕方法，典型夫妇可能不如参加临床试验的夫妇那样遵守规则。当违反三条最严格规则中的任何一条——黏液期无性交、生育高峰期当天过后三天内无性交或压力期无性交——时，意外怀孕的概率最大。那些对规则态度不端正的人更有可能冒险，包括冒严重风险。冒险成功（即未怀孕）的人很可能再次冒险。由于违反最严格规则每个周期会带来28%的怀孕风险，对于可能冒险的人，应该告知其可能的后果。
