Jo Ann Rosenfeld, MD

There are many highly effective contraceptive methods available. Some, including oral birth control pills (OCPs), injectable and implantable hormones, and sterilization of both sexes, have ideal effectiveness rates higher than 98% for preventing pregnancy.

However, contraception is not always used ideally. Unplanned or unintended pregnancies do occur. In 1988, US women aged 15 to 44 years reported that 35% of their full-term pregnancies in the preceding 5 years were unintended,¹ in some populations 60% of pregnancies were reported as unintended,² and one third of these ended in abortion.³ Sixty-five percent of adolescent pregnancies are also unintended.⁴ And more than 1 million pregnancies annually are reported to have occurred from misuse of OCPs.³

Pregnancies that occur while the woman is using contraception are considered unintended. These types of pregnancies have poorer outcomes when carried to term than other pregnancies, including an increased incidence of premature birth and intrauterine growth retardation.⁵ This is an area of concern for the women, their partners, and the health care providers who help these women with contraception concerns.

The way women use and experience contraception profoundly affects its effectiveness.^2,6 OCPs have been available for more than 30 years, implantable contraceptives for approximately 8 years, and injectable contraceptives for 5 to 7 years. Today, there are women who have had greater than 95% effective hormonal contraception available all their childbearing years. The purpose of our study was to discover what patterns of contraceptive use women developed during their childbearing years and how these patterns were related to unintended pregnancies.

Methods

We obtained a convenience sampling of women who visited an urban/suburban family practice residency office that is run by an open-panel health maintenance organization (HMO), but accepts more than 60 different health plans (including Medicare, HMO Medicare, Medicaid, HMO Medicaid, and self-pay) and includes maternity care. The practice has more than 25,000 patient visits annually (60% women) who are residents of eastern Baltimore, Maryland, and Baltimore County. Women patients and women relatives or friends of patients who entered the waiting room and were aged 18 to 50 years were asked to participate in a face-to-face half-hour interview by a medical student. Two medical students talked with 396 women during June and July 1999. The interviews were recorded by identification number only.

After giving informed consent, the women spent approximately 30 minutes answering questions about marital status, education, socioeconomic status, drug and cigarette use, and first and subsequent contraceptive experiences. Each woman was given a list of 20 methods of contraception and 20 reasons for discontinuation to help her; both lists had an “other” category. She was asked what contraceptive she remembered she had used first, for how long, and when she had changed methods and why; these questions were repeated for every method used since. The questionnaire had been pretested for 2 years in 2 previous studies of more than 600 women. Only 5 women refused to participate in our survey, most often because of time constraints.

One author (J.A.R.) entered the data in Excel (Microsoft Corporation; Redmond, Wash) spreadsheets. The statisticians at the MedAtlantic Research Institute converted the spreadsheets with the Statistical Package for the Social Sciences (SPSS, Inc; Chicago, Ill). Variables were analyzed for correlation and significance by Student t and chi-square tests. The patterns of contraceptive use were analyzed, and demographic values were compared.

Results

A total of 396 women participated in our survey. They were representative of urban/suburban women in general and of women of the practice [Table 1]. The average age of the respondents was 27 years; 98% (n=389) had sex with a man.

During their lifetimes these women used an aggregate total of 1421 methods of contraception (average=3.52±1.56 methods per woman; range=1-10). The methods used are listed in [Table 2]. All of the women (except the 5 who never had sex with a man) used 1 method of contraception at least once, 370 (93%) had used 2 methods, 287 (72%) had used 3, and 217 (55%) had used 4 or more. Eighty-one women (20%) reported having used more than 1 method at a time. Condoms were the most common first method of contraception (62%). The most common contraceptive methods used overall were OCPs (81%) and condoms (78%). Thirty-four percent had tried implantable or injectable hormonal contraception at least once. And 67 women (17%) had a tubal ligation (average age=28 years; range=21-45 years).

A total of 168 (42%) of the women became pregnant while using contraception, some more than once. Forty-nine women (13%) became pregnant twice and 13 women (3%) 3 times while using contraception. One hundred three women became pregnant while taking OCPs, 78 while using condoms and 11 while using an injectable contraceptive (depot medroxyprogesterone [DMP]). The women who became pregnant while using birth control were significantly more likely to be younger and African American. They were more likely to have a history of early initiation of birth control use, and they used more methods during their lifetimes. These women were pregnant more often and were more likely to use some type of public assistance [Table 3]. The total group of African American women in our survey was more likely to be younger, have some college education, and be single. Marital status did not correlate with becoming pregnant while using birth control; married, single, and separated or divorced women were equally represented. Women who became pregnant while using birth control were not more likely to use cigarettes, marijuana, or cocaine; have more partners; or start sexual relationships at an earlier age.

The rate of actual-use effectiveness of OCPs in this population was similar to the national average. First-year use effectiveness rates could not be determined, but lifetime effectiveness rates were estimated. Three hundred nineteen women (81%) had used OCPs for an aggregate total of 1422 years (average=4.5 years per woman; range=1 month-28 years). One hundred three women (33.3%) reported that they had become pregnant while using OCPs, some more than once. This was a pregnancy rate of 7.5%. Fifty-seven percent of the women who became pregnant while taking OCPs said they stopped using them because of getting pregnant.

Three hundred four women (78%) had used condoms for an aggregate total of 1178 years (average=3.9 years per woman; range=1 month-25 years). Seventy-eight women (25.6%) reported becoming pregnant while using condoms for a pregnancy rate of 6.6%. Fifty-five percent of those women stated that becoming pregnant was the reason for stopping condom use.

Eight-two women had used DMP for an aggregate total of 77.1 years with an average of 11 months per woman. Eleven women reported that they became pregnant while using DMP, for a pregnancy rate in our study of 14%.

Two major patterns of contraceptive use during a woman’s lifetime emerged. These 2 patterns described the contraceptive choices or directions of 82% of the women. One group of 210 women (53%), who will be called the “effective contraceptors,” started with condoms and then used OCPs or DMP. Following that change, they either continued to take OCPs or changed again to a method with a higher actual effectiveness rate (ie, DMP, Norplant [Wyeth-Ayerst; St. Davids, Penn] intrauterine device, tubal ligation, vasectomy, or hysterectomy).

The other group (the “less effective contraceptors,” n=110, 29%) also started with condoms. Forty-seven of these women changed immediately to a method less effective overall than OCPs (rhythm, withdrawal, gel/foam, diaphragm, or no method); 25 changed to use OCPs, and then began to use methods less effective than OCPs. Also included in this group were 38 women who began contraception by taking OCPs and then changed to less effective methods.

The effective contraceptors were significantly less likely to become pregnant while using birth control than the less effective contraceptors. Only 37% of the effective contraceptors became pregnant; 51% of the less effective contraceptors did so (odds ratio= 1.4; [Table 4]).

Discussion

Although there has been much research into the effectiveness, side effects, and reasons for discontinuation of individual birth control methods, the personal histories of how women have used contraception has seldom been examined. We attempted to document patterns of contraception use and to relate these patterns to unintended pregnancies.

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.

In previous studies, a physician’s involvement did not affect a woman’s use or satisfaction with contraception.¹¹ In our study most women had discussed their satisfaction with contraceptive methods with a physician. Discussion or lack of discussion did not affect methods chosen, number of methods chosen, or the chance of becoming pregnant while using birth control.

Other studies have suggested that women may choose highly effective contraception, especially irreversible contraception, because of fear of pregnancy, then have a more satisfying sexual life because this fear has been reduced.^12,13 However, in our study population, no form of contraception significantly affected a woman’s satisfaction with her sexual life.

There was a very low effectiveness rate in our population of women who used DMP. DMP is usually a very effective method that boasts ideal and actual failure rates of less than 3 in 1000 women-years (99.7%). However, 11 of the 82 women in our study reported that they became pregnant while using DMP, for an effectiveness rate of only 86%. The average duration of use of DMP in these women was only 11 months (range=1 week to 60 months). It cannot be determined from the data, but because the average length of use is so short many of these pregnancies may have occurred in women who received only 1 shot and never returned. They considered themselves users of DMP, even though the medication’s effectiveness had waned. Women may have also wanted to please the interviewer or give an answer they thought was appropriate. This may be a major bias of our method of obtaining data. It is perhaps more socially acceptable to claim to be using DMP than to admit to using no method at all, or to claim to be using condoms consistently when actually only using them occasionally.

Limitations

Our study has inherent difficulties. It was a convenience sample; women who were interviewed might have been visiting the physician to obtain a prescription method of contraception, while those who used over-the-counter or rhythm methods may not have been counted proportionally. There is also an inherent recall bias. Women may be more likely to remember a significant fact (such as a pregnancy) as the reason for changing a birth control method rather than the headaches or irregular bleeding that may have contributed to the change. The women may have been more likely to tell the interviewer they were using a birth control method that failed than to say they stopped or forgot to use their method. This would make the methods look less effective.

However, reasons for changing or using contraception are based on the women’s perceptions, so although recall bias may occur, the women’s perceptions are as important as the actual happenings. Whether a woman became pregnant while using a particular method was defined by her recall of the situation. No objective measurements (counting pills, checking charts for DMP shots) were performed. The woman’s perceptions were important for our study because they affected her subsequent use of contraceptive methods. This induced a bias, however, from the interpretation by the women. Another flaw of our study is that it was difficult to determine when a woman was using more than one method concurrently; this would give a higher effectiveness rate than either method individually and overall.

A prospective concurrent study of women’s use of contraception over time would give better answers about why women change contraception and how they use it.

Conclusions

Pregnancy must be considered a possible risk even for those women using an effective method of contraception over a lifetime. OCPs had a lifetime risk of one third for pregnancy in our study population. However, by determining a woman’s pattern of contraceptive use, the health care professional may be able to pinpoint some women who are at higher risk for unintended pregnancies. These women should be followed up more closely and urged to use more effective contraception.

There are many highly effective contraceptive methods available. Some, including oral birth control pills (OCPs), injectable and implantable hormones, and sterilization of both sexes, have ideal effectiveness rates higher than 98% for preventing pregnancy.

However, contraception is not always used ideally. Unplanned or unintended pregnancies do occur. In 1988, US women aged 15 to 44 years reported that 35% of their full-term pregnancies in the preceding 5 years were unintended,¹ in some populations 60% of pregnancies were reported as unintended,² and one third of these ended in abortion.³ Sixty-five percent of adolescent pregnancies are also unintended.⁴ And more than 1 million pregnancies annually are reported to have occurred from misuse of OCPs.³

Pregnancies that occur while the woman is using contraception are considered unintended. These types of pregnancies have poorer outcomes when carried to term than other pregnancies, including an increased incidence of premature birth and intrauterine growth retardation.⁵ This is an area of concern for the women, their partners, and the health care providers who help these women with contraception concerns.

The way women use and experience contraception profoundly affects its effectiveness.^2,6 OCPs have been available for more than 30 years, implantable contraceptives for approximately 8 years, and injectable contraceptives for 5 to 7 years. Today, there are women who have had greater than 95% effective hormonal contraception available all their childbearing years. The purpose of our study was to discover what patterns of contraceptive use women developed during their childbearing years and how these patterns were related to unintended pregnancies.

Methods

We obtained a convenience sampling of women who visited an urban/suburban family practice residency office that is run by an open-panel health maintenance organization (HMO), but accepts more than 60 different health plans (including Medicare, HMO Medicare, Medicaid, HMO Medicaid, and self-pay) and includes maternity care. The practice has more than 25,000 patient visits annually (60% women) who are residents of eastern Baltimore, Maryland, and Baltimore County. Women patients and women relatives or friends of patients who entered the waiting room and were aged 18 to 50 years were asked to participate in a face-to-face half-hour interview by a medical student. Two medical students talked with 396 women during June and July 1999. The interviews were recorded by identification number only.

After giving informed consent, the women spent approximately 30 minutes answering questions about marital status, education, socioeconomic status, drug and cigarette use, and first and subsequent contraceptive experiences. Each woman was given a list of 20 methods of contraception and 20 reasons for discontinuation to help her; both lists had an “other” category. She was asked what contraceptive she remembered she had used first, for how long, and when she had changed methods and why; these questions were repeated for every method used since. The questionnaire had been pretested for 2 years in 2 previous studies of more than 600 women. Only 5 women refused to participate in our survey, most often because of time constraints.

One author (J.A.R.) entered the data in Excel (Microsoft Corporation; Redmond, Wash) spreadsheets. The statisticians at the MedAtlantic Research Institute converted the spreadsheets with the Statistical Package for the Social Sciences (SPSS, Inc; Chicago, Ill). Variables were analyzed for correlation and significance by Student t and chi-square tests. The patterns of contraceptive use were analyzed, and demographic values were compared.

Results

A total of 396 women participated in our survey. They were representative of urban/suburban women in general and of women of the practice [Table 1]. The average age of the respondents was 27 years; 98% (n=389) had sex with a man.

During their lifetimes these women used an aggregate total of 1421 methods of contraception (average=3.52±1.56 methods per woman; range=1-10). The methods used are listed in [Table 2]. All of the women (except the 5 who never had sex with a man) used 1 method of contraception at least once, 370 (93%) had used 2 methods, 287 (72%) had used 3, and 217 (55%) had used 4 or more. Eighty-one women (20%) reported having used more than 1 method at a time. Condoms were the most common first method of contraception (62%). The most common contraceptive methods used overall were OCPs (81%) and condoms (78%). Thirty-four percent had tried implantable or injectable hormonal contraception at least once. And 67 women (17%) had a tubal ligation (average age=28 years; range=21-45 years).

A total of 168 (42%) of the women became pregnant while using contraception, some more than once. Forty-nine women (13%) became pregnant twice and 13 women (3%) 3 times while using contraception. One hundred three women became pregnant while taking OCPs, 78 while using condoms and 11 while using an injectable contraceptive (depot medroxyprogesterone [DMP]). The women who became pregnant while using birth control were significantly more likely to be younger and African American. They were more likely to have a history of early initiation of birth control use, and they used more methods during their lifetimes. These women were pregnant more often and were more likely to use some type of public assistance [Table 3]. The total group of African American women in our survey was more likely to be younger, have some college education, and be single. Marital status did not correlate with becoming pregnant while using birth control; married, single, and separated or divorced women were equally represented. Women who became pregnant while using birth control were not more likely to use cigarettes, marijuana, or cocaine; have more partners; or start sexual relationships at an earlier age.

The rate of actual-use effectiveness of OCPs in this population was similar to the national average. First-year use effectiveness rates could not be determined, but lifetime effectiveness rates were estimated. Three hundred nineteen women (81%) had used OCPs for an aggregate total of 1422 years (average=4.5 years per woman; range=1 month-28 years). One hundred three women (33.3%) reported that they had become pregnant while using OCPs, some more than once. This was a pregnancy rate of 7.5%. Fifty-seven percent of the women who became pregnant while taking OCPs said they stopped using them because of getting pregnant.

Three hundred four women (78%) had used condoms for an aggregate total of 1178 years (average=3.9 years per woman; range=1 month-25 years). Seventy-eight women (25.6%) reported becoming pregnant while using condoms for a pregnancy rate of 6.6%. Fifty-five percent of those women stated that becoming pregnant was the reason for stopping condom use.

Eight-two women had used DMP for an aggregate total of 77.1 years with an average of 11 months per woman. Eleven women reported that they became pregnant while using DMP, for a pregnancy rate in our study of 14%.

Two major patterns of contraceptive use during a woman’s lifetime emerged. These 2 patterns described the contraceptive choices or directions of 82% of the women. One group of 210 women (53%), who will be called the “effective contraceptors,” started with condoms and then used OCPs or DMP. Following that change, they either continued to take OCPs or changed again to a method with a higher actual effectiveness rate (ie, DMP, Norplant [Wyeth-Ayerst; St. Davids, Penn] intrauterine device, tubal ligation, vasectomy, or hysterectomy).

The other group (the “less effective contraceptors,” n=110, 29%) also started with condoms. Forty-seven of these women changed immediately to a method less effective overall than OCPs (rhythm, withdrawal, gel/foam, diaphragm, or no method); 25 changed to use OCPs, and then began to use methods less effective than OCPs. Also included in this group were 38 women who began contraception by taking OCPs and then changed to less effective methods.

The effective contraceptors were significantly less likely to become pregnant while using birth control than the less effective contraceptors. Only 37% of the effective contraceptors became pregnant; 51% of the less effective contraceptors did so (odds ratio= 1.4; [Table 4]).

Discussion

Although there has been much research into the effectiveness, side effects, and reasons for discontinuation of individual birth control methods, the personal histories of how women have used contraception has seldom been examined. We attempted to document patterns of contraception use and to relate these patterns to unintended pregnancies.

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.

In previous studies, a physician’s involvement did not affect a woman’s use or satisfaction with contraception.¹¹ In our study most women had discussed their satisfaction with contraceptive methods with a physician. Discussion or lack of discussion did not affect methods chosen, number of methods chosen, or the chance of becoming pregnant while using birth control.

Other studies have suggested that women may choose highly effective contraception, especially irreversible contraception, because of fear of pregnancy, then have a more satisfying sexual life because this fear has been reduced.^12,13 However, in our study population, no form of contraception significantly affected a woman’s satisfaction with her sexual life.

There was a very low effectiveness rate in our population of women who used DMP. DMP is usually a very effective method that boasts ideal and actual failure rates of less than 3 in 1000 women-years (99.7%). However, 11 of the 82 women in our study reported that they became pregnant while using DMP, for an effectiveness rate of only 86%. The average duration of use of DMP in these women was only 11 months (range=1 week to 60 months). It cannot be determined from the data, but because the average length of use is so short many of these pregnancies may have occurred in women who received only 1 shot and never returned. They considered themselves users of DMP, even though the medication’s effectiveness had waned. Women may have also wanted to please the interviewer or give an answer they thought was appropriate. This may be a major bias of our method of obtaining data. It is perhaps more socially acceptable to claim to be using DMP than to admit to using no method at all, or to claim to be using condoms consistently when actually only using them occasionally.

Limitations

Our study has inherent difficulties. It was a convenience sample; women who were interviewed might have been visiting the physician to obtain a prescription method of contraception, while those who used over-the-counter or rhythm methods may not have been counted proportionally. There is also an inherent recall bias. Women may be more likely to remember a significant fact (such as a pregnancy) as the reason for changing a birth control method rather than the headaches or irregular bleeding that may have contributed to the change. The women may have been more likely to tell the interviewer they were using a birth control method that failed than to say they stopped or forgot to use their method. This would make the methods look less effective.

However, reasons for changing or using contraception are based on the women’s perceptions, so although recall bias may occur, the women’s perceptions are as important as the actual happenings. Whether a woman became pregnant while using a particular method was defined by her recall of the situation. No objective measurements (counting pills, checking charts for DMP shots) were performed. The woman’s perceptions were important for our study because they affected her subsequent use of contraceptive methods. This induced a bias, however, from the interpretation by the women. Another flaw of our study is that it was difficult to determine when a woman was using more than one method concurrently; this would give a higher effectiveness rate than either method individually and overall.

A prospective concurrent study of women’s use of contraception over time would give better answers about why women change contraception and how they use it.

Conclusions

Pregnancy must be considered a possible risk even for those women using an effective method of contraception over a lifetime. OCPs had a lifetime risk of one third for pregnancy in our study population. However, by determining a woman’s pattern of contraceptive use, the health care professional may be able to pinpoint some women who are at higher risk for unintended pregnancies. These women should be followed up more closely and urged to use more effective contraception.

There are many highly effective contraceptive methods available. Some, including oral birth control pills (OCPs), injectable and implantable hormones, and sterilization of both sexes, have ideal effectiveness rates higher than 98% for preventing pregnancy.

However, contraception is not always used ideally. Unplanned or unintended pregnancies do occur. In 1988, US women aged 15 to 44 years reported that 35% of their full-term pregnancies in the preceding 5 years were unintended,¹ in some populations 60% of pregnancies were reported as unintended,² and one third of these ended in abortion.³ Sixty-five percent of adolescent pregnancies are also unintended.⁴ And more than 1 million pregnancies annually are reported to have occurred from misuse of OCPs.³

Pregnancies that occur while the woman is using contraception are considered unintended. These types of pregnancies have poorer outcomes when carried to term than other pregnancies, including an increased incidence of premature birth and intrauterine growth retardation.⁵ This is an area of concern for the women, their partners, and the health care providers who help these women with contraception concerns.

The way women use and experience contraception profoundly affects its effectiveness.^2,6 OCPs have been available for more than 30 years, implantable contraceptives for approximately 8 years, and injectable contraceptives for 5 to 7 years. Today, there are women who have had greater than 95% effective hormonal contraception available all their childbearing years. The purpose of our study was to discover what patterns of contraceptive use women developed during their childbearing years and how these patterns were related to unintended pregnancies.

Methods

We obtained a convenience sampling of women who visited an urban/suburban family practice residency office that is run by an open-panel health maintenance organization (HMO), but accepts more than 60 different health plans (including Medicare, HMO Medicare, Medicaid, HMO Medicaid, and self-pay) and includes maternity care. The practice has more than 25,000 patient visits annually (60% women) who are residents of eastern Baltimore, Maryland, and Baltimore County. Women patients and women relatives or friends of patients who entered the waiting room and were aged 18 to 50 years were asked to participate in a face-to-face half-hour interview by a medical student. Two medical students talked with 396 women during June and July 1999. The interviews were recorded by identification number only.

After giving informed consent, the women spent approximately 30 minutes answering questions about marital status, education, socioeconomic status, drug and cigarette use, and first and subsequent contraceptive experiences. Each woman was given a list of 20 methods of contraception and 20 reasons for discontinuation to help her; both lists had an “other” category. She was asked what contraceptive she remembered she had used first, for how long, and when she had changed methods and why; these questions were repeated for every method used since. The questionnaire had been pretested for 2 years in 2 previous studies of more than 600 women. Only 5 women refused to participate in our survey, most often because of time constraints.

One author (J.A.R.) entered the data in Excel (Microsoft Corporation; Redmond, Wash) spreadsheets. The statisticians at the MedAtlantic Research Institute converted the spreadsheets with the Statistical Package for the Social Sciences (SPSS, Inc; Chicago, Ill). Variables were analyzed for correlation and significance by Student t and chi-square tests. The patterns of contraceptive use were analyzed, and demographic values were compared.

Results

A total of 396 women participated in our survey. They were representative of urban/suburban women in general and of women of the practice [Table 1]. The average age of the respondents was 27 years; 98% (n=389) had sex with a man.

During their lifetimes these women used an aggregate total of 1421 methods of contraception (average=3.52±1.56 methods per woman; range=1-10). The methods used are listed in [Table 2]. All of the women (except the 5 who never had sex with a man) used 1 method of contraception at least once, 370 (93%) had used 2 methods, 287 (72%) had used 3, and 217 (55%) had used 4 or more. Eighty-one women (20%) reported having used more than 1 method at a time. Condoms were the most common first method of contraception (62%). The most common contraceptive methods used overall were OCPs (81%) and condoms (78%). Thirty-four percent had tried implantable or injectable hormonal contraception at least once. And 67 women (17%) had a tubal ligation (average age=28 years; range=21-45 years).

A total of 168 (42%) of the women became pregnant while using contraception, some more than once. Forty-nine women (13%) became pregnant twice and 13 women (3%) 3 times while using contraception. One hundred three women became pregnant while taking OCPs, 78 while using condoms and 11 while using an injectable contraceptive (depot medroxyprogesterone [DMP]). The women who became pregnant while using birth control were significantly more likely to be younger and African American. They were more likely to have a history of early initiation of birth control use, and they used more methods during their lifetimes. These women were pregnant more often and were more likely to use some type of public assistance [Table 3]. The total group of African American women in our survey was more likely to be younger, have some college education, and be single. Marital status did not correlate with becoming pregnant while using birth control; married, single, and separated or divorced women were equally represented. Women who became pregnant while using birth control were not more likely to use cigarettes, marijuana, or cocaine; have more partners; or start sexual relationships at an earlier age.

The rate of actual-use effectiveness of OCPs in this population was similar to the national average. First-year use effectiveness rates could not be determined, but lifetime effectiveness rates were estimated. Three hundred nineteen women (81%) had used OCPs for an aggregate total of 1422 years (average=4.5 years per woman; range=1 month-28 years). One hundred three women (33.3%) reported that they had become pregnant while using OCPs, some more than once. This was a pregnancy rate of 7.5%. Fifty-seven percent of the women who became pregnant while taking OCPs said they stopped using them because of getting pregnant.

Three hundred four women (78%) had used condoms for an aggregate total of 1178 years (average=3.9 years per woman; range=1 month-25 years). Seventy-eight women (25.6%) reported becoming pregnant while using condoms for a pregnancy rate of 6.6%. Fifty-five percent of those women stated that becoming pregnant was the reason for stopping condom use.

Eight-two women had used DMP for an aggregate total of 77.1 years with an average of 11 months per woman. Eleven women reported that they became pregnant while using DMP, for a pregnancy rate in our study of 14%.

Two major patterns of contraceptive use during a woman’s lifetime emerged. These 2 patterns described the contraceptive choices or directions of 82% of the women. One group of 210 women (53%), who will be called the “effective contraceptors,” started with condoms and then used OCPs or DMP. Following that change, they either continued to take OCPs or changed again to a method with a higher actual effectiveness rate (ie, DMP, Norplant [Wyeth-Ayerst; St. Davids, Penn] intrauterine device, tubal ligation, vasectomy, or hysterectomy).

The other group (the “less effective contraceptors,” n=110, 29%) also started with condoms. Forty-seven of these women changed immediately to a method less effective overall than OCPs (rhythm, withdrawal, gel/foam, diaphragm, or no method); 25 changed to use OCPs, and then began to use methods less effective than OCPs. Also included in this group were 38 women who began contraception by taking OCPs and then changed to less effective methods.

The effective contraceptors were significantly less likely to become pregnant while using birth control than the less effective contraceptors. Only 37% of the effective contraceptors became pregnant; 51% of the less effective contraceptors did so (odds ratio= 1.4; [Table 4]).

Discussion

Although there has been much research into the effectiveness, side effects, and reasons for discontinuation of individual birth control methods, the personal histories of how women have used contraception has seldom been examined. We attempted to document patterns of contraception use and to relate these patterns to unintended pregnancies.

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.

In previous studies, a physician’s involvement did not affect a woman’s use or satisfaction with contraception.¹¹ In our study most women had discussed their satisfaction with contraceptive methods with a physician. Discussion or lack of discussion did not affect methods chosen, number of methods chosen, or the chance of becoming pregnant while using birth control.

Other studies have suggested that women may choose highly effective contraception, especially irreversible contraception, because of fear of pregnancy, then have a more satisfying sexual life because this fear has been reduced.^12,13 However, in our study population, no form of contraception significantly affected a woman’s satisfaction with her sexual life.

There was a very low effectiveness rate in our population of women who used DMP. DMP is usually a very effective method that boasts ideal and actual failure rates of less than 3 in 1000 women-years (99.7%). However, 11 of the 82 women in our study reported that they became pregnant while using DMP, for an effectiveness rate of only 86%. The average duration of use of DMP in these women was only 11 months (range=1 week to 60 months). It cannot be determined from the data, but because the average length of use is so short many of these pregnancies may have occurred in women who received only 1 shot and never returned. They considered themselves users of DMP, even though the medication’s effectiveness had waned. Women may have also wanted to please the interviewer or give an answer they thought was appropriate. This may be a major bias of our method of obtaining data. It is perhaps more socially acceptable to claim to be using DMP than to admit to using no method at all, or to claim to be using condoms consistently when actually only using them occasionally.

Limitations

Our study has inherent difficulties. It was a convenience sample; women who were interviewed might have been visiting the physician to obtain a prescription method of contraception, while those who used over-the-counter or rhythm methods may not have been counted proportionally. There is also an inherent recall bias. Women may be more likely to remember a significant fact (such as a pregnancy) as the reason for changing a birth control method rather than the headaches or irregular bleeding that may have contributed to the change. The women may have been more likely to tell the interviewer they were using a birth control method that failed than to say they stopped or forgot to use their method. This would make the methods look less effective.

However, reasons for changing or using contraception are based on the women’s perceptions, so although recall bias may occur, the women’s perceptions are as important as the actual happenings. Whether a woman became pregnant while using a particular method was defined by her recall of the situation. No objective measurements (counting pills, checking charts for DMP shots) were performed. The woman’s perceptions were important for our study because they affected her subsequent use of contraceptive methods. This induced a bias, however, from the interpretation by the women. Another flaw of our study is that it was difficult to determine when a woman was using more than one method concurrently; this would give a higher effectiveness rate than either method individually and overall.

A prospective concurrent study of women’s use of contraception over time would give better answers about why women change contraception and how they use it.

Conclusions

Pregnancy must be considered a possible risk even for those women using an effective method of contraception over a lifetime. OCPs had a lifetime risk of one third for pregnancy in our study population. However, by determining a woman’s pattern of contraceptive use, the health care professional may be able to pinpoint some women who are at higher risk for unintended pregnancies. These women should be followed up more closely and urged to use more effective contraception.