Factors that affect contraceptive failure rates and probabilities reported in the literature can be usefully divided into three categories: 1 the inherent efficacy of the method when used correctly and consistently perfect use and the technical attributes of the method that facilitate or interfere with proper use; 2 characteristics of the user; and 3 competence and honesty of the investigator in planning and executing the study, and in analysing and reporting the results.
For some methods, such as sterilization, implants, the copper-T IUD and the LNG-IUS, the inherent efficacy is so high, and proper and consistent use is so nearly guaranteed, that extremely low pregnancy rates are found in all studies, and the range of reported pregnancy rates is quite narrow. For other methods such as the pill and injectable, inherent efficacy is high, but there is still room for potential imperfect use forgetting to take pills or failure to return on time for injections , so the second factor can contribute to a wider range of reported probabilities of pregnancy.
In general, the studies of sterilization, injectable, implant, pill, patch, ring, IUD and IUS use have been very competently executed and analysed. Studies of periodic abstinence, spermicides and the barrier methods display a wide range of reported probabilities of pregnancy because the potential for imperfect use is high, the inherent efficacy is relatively low, and the competence of the investigators is mixed.
Characteristics of the users can affect the pregnancy rate for any method under investigation, but the impact will be greatest when the pregnancy rates during typical use are highest, either because the method has less inherent efficacy or because it is hard to use consistently or correctly.
The user characteristic that is probably most important is imperfect use of the method. Investigators routinely separate the unintended pregnancies into two groups. By convention, pregnancies that occur during a month in which a method was used improperly are classified as user failures even though, logically, a pregnancy may be due to failure of the method, if it was used correctly on some occasions and incorrectly on others , and all other pregnancies are classified as method failures.
However, investigators do not separate the exposure the denominator in the calculation of failure rates into these two groups. Since investigators do not generally inquire about perfect use except when a pregnancy occurs, the proper calculations cannot be performed. The importance of perfect use is demonstrated in the few studies where the requisite information on quality of use was collected.
For example, in a World Health Organization study of the ovulation method, the proportion of women becoming pregnant among those who used the method perfectly during the first year was 3. Among those who use a method consistently and correctly perfect users , the most important user characteristic that determines the risk of pregnancy is frequency of intercourse.
For example, in a study in which users were randomly assigned to either the diaphragm or the sponge, diaphragm users who had intercourse four or more times a week became pregnant in the first year twice as frequently as those who had intercourse fewer than four times a week. This decline is likely to be pronounced among those who are routinely exposed to sexually transmitted infections such as chlamydia and gonorrhoea.
Among less-exposed women, the decline is likely to be moderate until a woman reaches her late thirties. The competence and honesty of the investigator also affect the published results. The errors committed by investigators range from simple arithmetical mistakes to outright fraud. In a two-page article published in the American Journal of Obstetrics and Gynecology , a first-year probability of pregnancy of 1.
Furthermore, he never subsequently revealed except to the A. Robins Company, which bought the shield from the Dalkon Corporation but did not reveal this information either that as the original trial matured, the first-year probability of pregnancy more than doubled. The system of drug testing in the USA, which demands that the company wishing to market a drug be responsible for conducting studies to assess its efficacy and safety, provides incentives for the unscrupulous to present less-than-honest results.
Some actions that are not deliberately dishonest are, nevertheless, not discouraged by the incentives in the present system. For example, a woman who becomes pregnant may be discarded from a clinical trial if the researcher decides that she did not fit the protocols after all. Or one can be less than vigilant in trying to contact patients lost to follow-up LFU.
The standard assumption made at the time of analysis is that women who are LFU experience unintended pregnancy at the same rate as those who are observed. This assumption is probably innocuous when the proportion LFU is small. For example, one study found that the pregnancy rate for calendar rhythm rose from 9. Several methodological pitfalls can snare investigators. One of the most common is a misleading measure of contraceptive failure called the Pearl index, which is obtained by dividing the number of unintended pregnancies by the number of years of exposure to the risk of unintended pregnancy contributed by all women in the study.
This measure can be misleading when one wishes to compare pregnancy rates obtained from studies with different average amounts of exposure. The likelihood of pregnancy declines over time because those most likely to become pregnant do so at earlier durations of contraceptive use and exit from observation. Which investigator is incorrect? The two rates are simply not comparable. In contrast, life-table measures of contraceptive failure are easy to interpret and control for the distorting effects of varying durations of use.
Another problem occurs when deciding which pregnancies to count. Most studies only count the pregnancies observed and reported by the women. If, on the other hand, a pregnancy test were administered every month, the number of pregnancies and hence the pregnancy rate would increase because early fetal losses not observed by the women would be added to the number of observed pregnancies.
Such routine pregnancy testing in the more recent contraceptive trials has resulted in higher pregnancy rates than would otherwise have been obtained, and makes the results non-comparable with those from other trials.
Other, more technical, errors that have biased reported results are discussed elsewhere. The incentives to conduct research on contraceptive failure vary widely between methods. Many studies of the pill and IUD exist because companies wishing to market them must conduct clinical trials to demonstrate their efficacy.
In contrast, few studies of withdrawal exist because there is no financial reward for investigating this method. Moreover, researchers face differing incentives to report unfavourable results. The vasectomy literature is filled with short articles by clinicians who have performed , or vasectomies. Surgeons with high pregnancy rates simply do not write articles calling attention to their poor surgical skills.
Likewise, drug companies do not commonly publicize their failures. Even if investigators prepared reports describing failures, journal editors would not be likely to publish them. Male and female sterilization and the long-acting reversible contraceptives the implant, injectable, IUD and IUS that do not require adherence are the most effective methods for protecting against pregnancy, but they offer no protection against sexually transmitted infections.
Hormonal methods requiring daily, weekly or monthly adherence are equally effective during typical use. Barrier methods are much less effective during typical use. Whether or not this differential is due to self-selection those most determined to avoid pregnancy choose methods with higher inherent efficacy is unknown. Most methods have a low risk of failure if they are used correctly and consistently. The most effective methods are those not requiring adherence.
Even a low annual risk of pregnancy implies a high cumulative risk of pregnancy during a lifetime of use. The most effective method for an individual woman or couple is a method that actually will be used correctly and consistently.
Emergency contraception offers a last chance to prevent pregnancy after unprotected intercourse. How can we improve our understanding of why men and women use contraceptives imperfectly, or not at all? What interventions would decrease misperceptions about contraceptive methods among users and providers?
How can various forms of media be used to communicate about contraceptive effectiveness, including the difference between typical use and perfect use? What interventions would increase use of methods with low typical-use failure rates long-acting reversible methods of contraception, such as intrauterine contraceptives and implants? How can we gain a better understanding of why women do not use emergency contraception when they have unprotected intercourse but do not want to become pregnant?
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
National Center for Biotechnology Information , U. Author manuscript; available in PMC Apr Author information Copyright and License information Disclaimer. Copyright notice. See other articles in PMC that cite the published article. Abstract Contraceptive failure is a major source of unintended pregnancy. Keywords: contraception, contraceptive failure.
Open in a separate window. Figure 1. Lactational amenorrhoea method is a highly effective, temporary method of contraception. The Standard Days method avoids intercourse on cycle days 8— Inherent efficacy For some methods, such as sterilization, implants, the copper-T IUD and the LNG-IUS, the inherent efficacy is so high, and proper and consistent use is so nearly guaranteed, that extremely low pregnancy rates are found in all studies, and the range of reported pregnancy rates is quite narrow.
User characteristics Characteristics of the users can affect the pregnancy rate for any method under investigation, but the impact will be greatest when the pregnancy rates during typical use are highest, either because the method has less inherent efficacy or because it is hard to use consistently or correctly.
Imperfect use The user characteristic that is probably most important is imperfect use of the method. Frequency of intercourse Among those who use a method consistently and correctly perfect users , the most important user characteristic that determines the risk of pregnancy is frequency of intercourse. Influence of the investigator The competence and honesty of the investigator also affect the published results.
Methodological pitfalls Several methodological pitfalls can snare investigators. Check out these best-sellers and special offers on books and newsletters from Mayo Clinic Press. This content does not have an English version. This content does not have an Arabic version. Sections for Birth control patch About. Overview Birth control patch Open pop-up dialog box Close. Birth control patch The birth control patch is a contraceptive device that contains the hormones estrogen and progestin.
Request an Appointment at Mayo Clinic. Share on: Facebook Twitter. Show references Birth control methods. Office on Women's Health. Accessed April 15, Xulane prescribing information. Mylan Pharmaceuticals; Accessed April 14, Burkman RT. Contraception: Transdermal contraceptive patches. Neither the owners or employees of Contracept.
Always speak with your primary health care provider before engaging in any form of self treatment. Please see our Legal Statement for further information.
All rights reserved. Use of this site constitutes acceptance of Contracept. The material on this site is for informational purposes only, and is not a substitute for medical advice, diagnosis or treatment provided by a qualified health care provider.
0コメント