Two patterns of lifetime contraceptive behaviors in women emerged in our study. Approximately half of the women in our study showed a pattern of changing their birth control methods to more effective ones (the effective contraceptors), and approximately one fourth chose a pattern of methods that became increasingly less effective (the less effective contraceptors). Thus, it may be possible for a health care professional to be able to place a patient in a low- or high-risk group for unintended pregnancy by asking a few questions about her contraception history. It may not ever be possible to completely determine prospectively who is at risk for unintended pregnancy since even highly effective methods have inherent pregnancy rates over time. Physicians, however, should start considering a woman’s history of contraceptive methods as a primary tool for helping to prevent unintended pregnancy.
Ideal Versus Actual Effectiveness
Effectiveness of the ideal use of contraceptive methods is determined by the number of pregnancies per 100 women using the method for 1 year. OCPs (98.5%), DMP (99.7%), Norplant (99.7%), tubal ligations (99.8%), vasectomy (99%), and hysterectomy (100%) all have high ideal-use effectiveness rates.3,7 These percentages make pregnancy while using any of these methods appear extremely unlikely. Condoms have an ideal effectiveness rate of 85% to 90%, still fairly high numbers.
However, contraception is rarely used ideally. Women forget to take a pill, forget to come in for a DMP shot, use condoms improperly, or become worried about side effects and suddenly stop using their chosen method. These mistakes are taken this into account by recording actual-use effectiveness rates. Still, OCPs have an actual effectiveness rate of 94% and condoms of 82% to 85%, and the actual effectiveness rates of tubal ligation and Norplant do not decrease from their ideal rates.3
Most women have used many forms of contraception, and in our group many changed several times during their lives. The average number of methods was more than 3, and more than half of the women used 5 or more methods. Many women had used both condoms and OCPs during their lives. The reasons for these changes should be examined more carefully (and possibly prospectively). Changing to a more effective method may indicate a lessened desire to ever become pregnant. Two studies have shown that women who want to postpone pregnancy are more likely to get pregnant than those who definitely do not want to give birth.8,9 Women who are dissatisfied with a contraceptive method are more likely to change that method and more likely to have an unintended pregnancy.2 Women who use the same contraceptive method for a long period are known to use them more efficiently.10 Similarly, women in our study who used several methods were more likely to have become pregnant while using birth control; older women were less likely to become pregnant.
Many women became pregnant while using methods that have good to excellent effectiveness rates. Although some admitted missing a pill or incorrectly using a condom, these women still felt they became pregnant while using a contraceptive method. Women taking OCPs had an actual effectiveness rate of 92.5%, close to the national average of 94%.3 Women using condoms had an effectiveness rate of 93.4%, much higher than the national average of 85%.3 This latter may be explained by the more recent trend of using condoms in addition to another method (condoms to prevent sexually transmitted diseases and another method for birth control).
Forty-two percent of the women in our study became pregnant while using contraception. One reason for this apparent paradox of high effectiveness rates and a high number of unintended pregnancies is the lifetime use of contraception. Effectiveness rates are calculated as the use of a method by 100 women for 1 year. These women all used contraception for more than 1 year. It is the natural history of OCPs with a 95% effectiveness rate that 1 in 20 women will get pregnant in 1 year, and 50 pregnancies will occur in 10 years. Usual actual use of high-effectiveness contraception still carries a significant risk of pregnancy that can be seen by the results in this population of women.
Becoming pregnant even while using good methods of birth control must be expected, explained, and understood. Physicians and their patients must not consider these pregnancies to be failures of the method used, but as inherent part of a life history of contraceptive use. Unintended pregnancies may be a consequence of using even very effective contraceptive methods.
Changing Methods
In our study, becoming pregnant while using a contraceptive method was very likely to cause the woman to discontinue using that method. More than half of the women who became pregnant while using contraception stated pregnancy was their primary reason for changing methods.