Review articleContraceptive failure in the United States
Introduction
Four pieces of information about contraceptive efficacy would help couples to make an informed decision when choosing a contraceptive method:
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Pregnancy rates during typical use show how effective the different methods are during actual use (including inconsistent or incorrect use).
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Pregnancy rates during perfect use show how effective methods can be, where perfect use is defined as following the directions for use.
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Pregnancy rates during imperfect use show how ineffective methods will be if they are used incorrectly or inconsistently. Pregnancy rates can be computed separately for different categories of imperfect use to reveal which types of imperfect use are most risky [1].
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The percentage of perfect users or percentage of months during which a method is used perfectly reveals how hard it is to use a method correctly and consistently.
The difference between pregnancy rates during imperfect use and pregnancy rates during perfect use reveals how forgiving of imperfect use a method is. The difference between pregnancy rates during typical use and pregnancy rates during perfect use reveals the consequences of imperfect use; this difference depends both on how unforgiving of imperfect use a method is and on how hard it is to use that method perfectly. Only the first two pieces of information are currently available. Our current understanding of the literature on contraceptive efficacy is summarized in Table 1.
In the column 2 of Table 1, we provide the estimates of the probabilities of pregnancy during the first year of typical use of each method in the United States. This information is shown graphically in Fig. 1 in a way that clients may find more useful [2]. For most methods, these estimates were derived from the experience of women in the 1995 National Survey of Family Growth (NSFG) [3] or the 1995 and 2002 NSFGs [3], [4], so that the information pertains to nationally representative samples of users. For the other methods, we based the estimates on evidence from surveys and clinical investigations. Pregnancy rates during typical use reflect how effective methods are for the average person who does not always use methods correctly or consistently. Typical use does not imply that a contraceptive method was always used. In the NSFG and in most clinical trials, a woman is “using” a contraceptive method if she considers herself to be using that method. Therefore, typical use of the condom could include actually using a condom only occasionally, and a woman could report that she is “using” the pill even though her supplies ran out several months ago. In short, “use”—which is identical to “typical use”—is a very elastic concept that depends entirely on an individual woman's perception.
In column 3 of Table 1, we provide our best guess of the probabilities of method failure (pregnancy) during the first year of perfect use. A method is used perfectly when it is used consistently according to a specified set of rules. For many methods, perfect use requires use at every act of intercourse. Virtually all method failure rates reported in the literature have been calculated incorrectly and are too low (see the discussion of methodological pitfalls below). Hence, we cannot empirically justify our estimates except those for four fertility awareness-based methods [1], [5], [6], [7], the diaphragm [8], the sponge, [8] the male condom [9], [10], [11], the female condom [12], spermicides [13] and methods for which there are extensive clinical trials with very low pregnancy rates. Even the estimates for the fertility awareness-based methods, female condom, diaphragm, spermicides and sponge are based on only one or two studies. Our hope is that our understanding of efficacy during perfect use for these and other methods will be enhanced by additional studies.
Column 4 of Table 1 displays the first-year probabilities of continuing use. They are based on the same sources used to derive the estimates in the second column (typical use).
Section snippets
No method
Our estimate of the percentage of women becoming pregnant among those not using contraception is based on populations in which the use of contraception is rare and on couples who report that they stopped using contraceptives because they want to conceive [14]. Based on this evidence, we conclude that 85 of 100 sexually active couples would experience a pregnancy in the first year if they used no contraception. Available evidence in the United States suggests that only about 40% of married
Typical use of spermicides, withdrawal, fertility awareness-based methods, diaphragm, male condom, oral contraceptive pills and Depo-Provera
Our estimates of the probability of pregnancy during the first year of typical use for withdrawal, fertility awareness-based methods, the male condom, the pill and Depo-Provera are taken from the 1995 and 2002 NSFG (the weighted average of the two estimates) and for spermicides and the diaphragm from the 1995 NSFG, all corrected for underreporting of abortion [3], [4].
The correction for underreporting of abortion may produce estimates that are too high because women in abortion clinics (surveys
Perfect use of the sponge and diaphragm
Our estimates of the probabilities of pregnancy during the first year of perfect use of the sponge and diaphragm correspond with results of a reanalysis of data from two clinical trials in which women were randomly assigned to use the diaphragm or sponge or to use the diaphragm or cervical cap [8]. The results indicate that among parous women who use the sponge perfectly, 19.4% to 20.5% will experience a pregnancy within the first year. The corresponding range for nulliparous women is 9.0% to
Typical use of the sponge
Here we also draw on results of the same clinical trial in which women were randomly assigned to the sponge or diaphragm [18]. The proportion becoming pregnant during the first year of typical use for parous users of the sponge (27.4%) was about twice as high as for nulliparous users of that method (14.0%). There was no such differential for the diaphragm, where the proportion becoming pregnant in the first year of typical use for parous users (12.4%) was marginally lower than that for
Female condom
The typical-use estimate for the female condom is based on the results of a 6-month clinical trial of the Reality female condom (now called the fc female condom); 12.4% of women in the United States experienced a pregnancy during the first 6 months of use [18]. The 12-month probability of pregnancy for users of Reality in the United States was projected from the relation between the pregnancy rates in the first 6 months and the pregnancy rates in the second 6 months for users of the diaphragm,
Perfect use of withdrawal and spermicides
Our estimate of the proportion of women becoming pregnant during a year of perfect use of withdrawal is a guess based on the reasoning that the risk of pregnancy resulting from pre-ejaculatory fluid is modest. Although three studies found no motile sperm in the pre-ejaculate [19], [20], [21], the most recent study did not replicate this result, perhaps because the samples were examined within 2 min of production [22]. In that study, 37% of subjects produced pre-ejaculatory samples that
Perfect use of fertility awareness-based methods
The perfect-use estimates for fertility awareness-based methods are based on empirical estimates of 4.8% for the Standard Days method [5], 3.5% for the TwoDay method [6], 3.2% for the ovulation method [1] and 0.4 per 100 women-years for the symptothermal method [7]. Published “method failure” rates for other variants of natural family planning are incorrect, because exposure includes all use, not just perfect use [1].
Perfect use of the male condom
Our estimate of the proportion of women becoming pregnant during a year of perfect use of the male condom is based on results from the only three studies of the male condom meeting modern standards of design, execution and analysis [9], [10], [11]. In each study, couples were randomly assigned to use either a latex condom or a polyurethane condom. All three studies reported efficacy during consistent use, but only one reported efficacy during perfect use [10]; in that study, the 6-cycle
Perfect use of oral contraceptive pills, Depo-Provera, and Implanon and typical use of Implanon
Although the lowest reported pregnancy rate for the combined pill during typical use is 0% [23], [24], recent studies indicate that pregnancies do occur, albeit rarely, during perfect use [25], [26]. Hence, we set the perfect-use estimate for the pill at the very low level of 0.3%. The lowest reported pregnancy rate for the progestin-only pill exceeds 1% [27], [28]. It is likely that the progestin-only pill is less effective than the combined pill during typical use, since the progestin-only
Evra and NuvaRing
The typical- and perfect-use estimates for the Evra patch and NuvaRing were set equal to those for the pill. It is possible that the patch and ring will prove to have better efficacy than the pill during typical use, because of better adherence with the dosing schedule. However, such superior efficacy has not been demonstrated in randomized trials. While in one trial, the failure rate was lower among women randomly assigned to use the Evra patch (1.2%) than among those assigned to use the pill
Intrauterine contraceptives
The estimate for typical use of the ParaGard (Copper T 380A) IUC, 0.8%, is taken directly from the largest study for that method [56]. The estimate for Mirena (levonorgestrel [LNG]-IUC), 0.2%, is the weighted average of the results from three studies [57], [58], [59]. The estimate for perfect use of the Copper T 380A, 0.6%, was obtained by removing the pregnancies that resulted when the device was not known to be in situ [60], on the perhaps questionable assumption that these pregnancies should
Sterilization
The weighted average of the results from nine vasectomy studies analyzed with life-table procedures is 0.02% of women becoming pregnant in the year following the procedure [61], [62], [63], [64], [65], [66], [67], [68], [69]. In eight of these studies, pregnancies occurred after the ejaculate had been declared to be sperm-free. This perfect-use estimate of 0.02% is undoubtedly too low, because clinicians are understandably loath to publish articles describing their surgical failures and
Contraceptive continuation
Contraceptives will be effective at preventing unintended pregnancy only if women or couples continue to use. The proportions of women continuing use at the end of the first year for withdrawal, fertility awareness-based methods, the male condom, the pill and Depo-Provera were obtained from the 2002 NSFG, and those for spermicides, the sponge and the diaphragm were obtained from the 1995 NSFG [16], [17]. Only method-related reasons for discontinuation (changing methods or termination of
The lactational amenorrhea method
The lactational amenorrhea method (LAM) is a highly effective, temporary method of contraception. If the infant is being exclusively breast-fed (or is given supplemental non-breast milk or pumped breast milk feeds only to a minor extent) and if the woman has not experienced her first postpartum menses, then breast-feeding provides more than 98% protection from pregnancy in the first 6 months following a birth [71], [72]. Four prospective clinical studies of the contraceptive effect of LAM
Conclusion
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Pregnancy rates during perfect use reflect how effective methods can be in preventing pregnancy when used consistently and correctly according to instructions.
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Pregnancy rates during typical use reflect how effective methods are for the average person who does not always use methods correctly or consistently.
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Pregnancy rates during typical use of adherence-dependent methods generally vary widely for different groups using the same method, primarily due to differences in the propensity to use the
Acknowledgments
This article is adapted from Trussell and Guthrie [77] and Trussell [14] (with the permission of Contraceptive Technology Communications, Inc.).
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