(Published June 2006)
Understanding Risk: Relative Effectiveness of Hormonal Contraceptives
Risk calculations are commonly used in reproductive health to present information on the relative effectiveness of contraceptives in preventing pregnancy. Estimates are based on several types of studies and are generally presented as the contraceptive failure rate, or the percentage of women likely to become pregnant while using a particular method for one year1 Generally, the rate of failure with typical use is presented along with the “lowest expected” or “perfect use” rate, as shown in Figure 8.
|FIGURE 8. Relative Effectiveness of Contraceptives1|
|The following table provides estimates of the percent of women likely to become pregnant while using a particular contraceptive method for one year. These estimates are based on a variety of studies.
“Typical Use” rates mean that the method either was not always used correctly or was not used with every act of sexual intercourse (e.g. sometimes forgot to take a birth control pill as directed and became pregnant) or was used correctly but failed anyway.
“Lowest Expected” rates mean that the method was always used correctly with every act of sexual intercourse but failed anyway (e.g. always took a birth control pill as directed but still became pregnant).
|Method||Typical Use Rate of Pregnancy||Lowest Expected Rate of Pregnancy|
Implant (Norplant and Norplant-2)
Hormone shot (Depo-Provera)
Combined Pill (Estrogen/Progestin)
Minipill (Progestin only)
|Intrauterine Devices (IUDs):
Male Latex Condom*
Vaginal Sponge (no previous births)
***Vaginal Sponge (previous births)
***Cervical Cap (no previous births)
**Cervical Cap (previous births)
|Spermicide: (gel, foam, suppository, film)||26%||6%|
Natural Family Planning (calendar, temperature,
* Used without Spermicide
** Used with Spermicide
Although tables comparing contraceptive failure rates for various methods are widely used, they may not be the most effective means for communicating about the risk of contraceptive failure. Steiner and colleagues from Family Health International and the Office of Population Research at Princeton provided women with three different tools for understanding contraceptive effectiveness: a numerical table similar to that shown in Figure 8, a categories table that separates contraceptive methods into three effectiveness categories (most effective for most users, effective for most users, less effective for most users), and a table that included both categories and numerical data.2
The researchers evaluated the improvement in knowledge about the relative effectiveness of two combinations of contraceptives: OCs versus hormonal injection and OCs versus condoms. They found that the categories chart improved knowledge the most and was statistically better than the numerical table, as shown in Table 2. This is probably because the categories chart organizes information around dimensions that are relevant to someone deciding between contraceptive methods, namely, the relative effectiveness of the methods.
However, before viewing the numerical table, the women tended to overestimate the absolute risk of pregnancy in a given effectiveness category. For example, 40 percent of women thought that the annual risk of pregnancy with a “very effective” method was 15 percent or greater, when in fact it is 0 percent to 1 percent, as defined by the World Health Organization (WHO).1 For this reason, the researchers suggested creating a categories table with a general range of risk within each category to provide an understanding of both the relative and absolute risk of pregnancy in each effectiveness category. WHO has developed a chart with illustrations of each contraceptive method that lists contraceptive effectiveness categories, along with actions to increase effectiveness of each method.2 This chart, which is shown in Figure 9, is available at: www.arhp.org/WHOchart.
- Steiner MJ, Dalebout S, Condon S, Dominik R, Trussell J. Understanding risk: a randomized controlled trial of communicating contraceptive effectiveness. Obstet Gynecol. 2003;102:709-17.
- World Health Organization. Comparing typical effectiveness of contraceptive methods. [Job Aid]. Geneva: World Health Organization, 2006. Available at: www.fhi.org/nr/shared/enFHI/Resources/EffectivenessChart.pdf. Accessed January 26, 2006.