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Pregnancy Probabilities During Use of the Creighton Model Fertility Care System
Margaret P. Howard, CNFPE;
Joseph B. Stanford, MD, MSPH
Arch Fam Med. 1999;8:391-402.
ABSTRACT
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Objective To evaluate pregnancy probabilities during use of the Creighton Model Fertility Care System (CrMS).
Design Couples who began use of the CrMS were entered into this observational cohort study. Follow-up included detailed reviews of use of the CrMS. Pregnancy probabilities were calculated with both net and gross life-table analysis through 18 months.
Setting A natural family planning service delivery program based at an urban hospital in Houston, Tex.
Subjects A group of 701 couples who received instruction in the CrMS were entered into the study. Most couples (93%) were engaged or married. Most women were white (83%), between the ages of 20 and 34 years (88%), and college graduates (58%).
Main Outcome Measure Pregnancies were classified based on a detailed evaluation involving the pregnant woman (usually with her partner).
Results At 12 months, the following net pregnancy probabilities were found per 100 couples: method-related pregnancies, 0.14; pregnancies caused by user and/or teacher error, 2.72; pregnancies caused by achieving-related behavior (genital contact during a time known to be fertile), 12.84; unresolved pregnancies, 1.43; and total pregnancies, 17.12. Pregnancy probabilities were similar when stratified by the following reproductive categories: uncomplicated regular cycles, long cycles, discontinuing oral contraceptives, breastfeeding, and other.
Conclusions Pregnancy probabilities of the CrMS compare favorably with those of other methods of family planning. Most pregnancies result from genital contact during a known fertile time. Women need not have regular cycles to use the CrMS successfully.
INTRODUCTION
PERIODIC ABSTINENCE to avoid pregnancy has been in use since "calendar rhythm" was developed by Ogino and Knaus1 in the 1930s. Unfortunately, the effectiveness of calendar rhythm to avoid pregnancy is, under optimal circumstances, only 85% to 94%.2 In the past few decades, methods of fertility awareness have been developed, including the ovulation method, which is based solely on women's observations of the vaginal discharge of cervical mucus, and the symptothermal method, which combines mucus observations with the measurement of basal body temperature and some calendar calculations. These methods can be referred to collectively as modern methods of natural family planning (NFP). Because modern methods of NFP are based on actual physiologic signs of fertility and infertility rather than calendar calculations, there are compelling reasons to believe that modern NFP can be more effective than calendar rhythm.3
Nevertheless, the effectiveness of modern methods of NFP to avoid pregnancy is still a subject of controversy.4-6 Effectiveness studies of the ovulation method have yielded method-related pregnancy rates of 0 to 11.3 pregnancies per 100 woman-years and total pregnancy rates of 0.4 to 39.7.7-8 For the symptothermal method, method-related pregnancy rates have been reported ranging from 0 to 13.1 and total pregnancy rates ranging from 3.3 to 34.4.8-9
It is apparent that there is wide variation in pregnancy rates among NFP studies. Some factors contributing to this variation are common to all family planning studies, including differences in the demographics and potential for fertility among the populations studied and differences in time contributed by study subjects.10 Also, while life-table analysis is a more appropriate method for analyzing pregnancies, most NFP studies have reported Pearl rates. Life-table probabilities are based on the proportion of subjects who become pregnant over a specified period, whereas Pearl rates are calculated based on the total number of cycles or months in a study.11 Pearl rates can vary substantially based on the time each subject is in the study. A detailed review of Pearl rates and life-table probabilities designed for clinicians is available elsewhere.12
Other important factors that contribute to variation in results among NFP studies stem from issues that are unique to NFP as opposed to other methods of family planning. There are 3 areas of particular importance: First, there is variability in the quality and standardization of training for NFP teachers and for teaching procedures, causing variability in the type of instruction received by NFP users (study subjects). The successful use of NFP, more than other methods of family planning, requires reliable instruction about proper use, including appropriate observation of the symptoms and signs of fertility, accurate interpretation of these signs, and support for questions and concerns. Second, the exclusion by nearly all NFP studies of women who do not have regular menstrual cycles makes it impossible to generalize results beyond women who have regular menses. Finally, NFP can be used either to avoid or to achieve pregnancy11, 13; unlike other methods of family planning, the choice to achieve a pregnancy does not presuppose discontinuation of the use of NFP. Hence, the evaluation of the choices of NFP users is a centrally important although controversial issue.6, 14-20 In NFP studies, most pregnancies result from intercourse on a day that has been identified as fertile by the method under study.6, 21-23
The Creighton Model Fertility Care System (CrMS) system was developed as a standardized version of the ovulation method originally described by Billings and Billings.24 The CrMS is characterized by rigorous professional training for teachers and standardized instruction procedures for clients learning the method, including a uniform recording system for vaginal discharges.25-28 Frequent follow-up and support is provided routinely during the first year of use of the CrMS. This report evaluates the pregnancy probabilities among couples served by a large US center providing CrMS instruction.
METHODS
This study is based on clients served by the St Joseph Hospital Natural Family Planning service program in Houston, Tex, from 1983 to 1989. During this time, the predominant type of NFP taught in this program was the CrMS. All new users of the CrMS were considered eligible for this study. Users were defined as clients who attended at least 1 follow-up session to learn the CrMS. Excluded were couples with a history of infertility, women who were pregnant at the time they began use of the CrMS, and women who were not genitally active at the time they began use of the CrMS. No attempt was made to select for couples who expressed strong motivation to avoid pregnancy for a time.
All subjects were entered into the study at the time the woman began charting her cycle. Unlike some other NFP studies,29-31 there was no preceding or separate learning phase for this study. Each couple contributed data until they had completed 18 months of follow-up, the woman became pregnant, or the couple left the study for some other reason (as detailed below). At entry to the study, all women were classified into one of the following reproductive status categories: (1) uncomplicated regular cycles, defined as 21 to 38 days32-33; (2) long cycles, defined as usually more than 38 days; (3) age 40 years or older; (4) having discontinued use of oral contraceptives within the past year; (5) totally breastfeeding; (6) breastfeeding/weaning (defined as having begun any type of supplemental feeding while still breastfeeding); (7) within 3 menstrual cycles after birth and not breastfeeding; or (8) within 1 cycle after an abortion (induced or spontaneous). While these categories are not necessarily mutually exclusive, for the purposes of this study they were treated as such; each woman was placed into the highest-numbered category that applied to her. For example, a woman aged 41 years who stopped taking oral contraceptives 11 months before study entry and was exclusively breastfeeding a baby born 1 month before study entry would have been listed as totally breastfeeding at the beginning of this study.
Detailed description of the CrMS instruction has been given elsewhere.25-28,34 After couples attended a group introductory session (occasionally the woman alone attended the introductory session), they began immediately to chart the woman's daily observations of vaginal discharge. According to standard CrMS protocol, women were asked to wipe across the vulva with flat white toilet tissue each time they use the bathroom to check for any discharge and to record its characteristics (degree of stretch, color, and whether it felt lubricative) according to a standardized recording system. Women were taught to make external observations only (ie, to not insert fingers into the vagina). Couples were instructed to abstain from genital contact for the first month of observations, but not all couples followed this instruction. Two weeks after the introductory session, couples (or sometimes women alone) received their first follow-up visit by meeting individually with the Creighton Model teacher, called an NFP practitioner. Over the following year, up to 8 follow-up visits took place. Generally these lasted 45 to 60 minutes each. During these visits, the NFP practitioner reviewed the couple's understanding of the CrMS using a standardized list of assessment items, gave instruction tailored to the individual circumstances, and reviewed all charting completed by the woman/couple for accuracy, completeness, and understanding of the times of fertility and infertility. In general, the time of fertility begins at the first appearance of mucus discharge and ends at the end of the fourth day after the peak day, defined in the CrMS as the last day mucus is clear, stretches more than 1 inch, or feels lubricative. The average duration of mucus discharge is 5 to 6 days. The CrMS also has special instructions to deal with breastfeeding, other oligo-ovulatory states, and chronic vaginal discharges. These are described in more detail elsewhere.25-27
Couples were explicitly asked at follow-up visits about any use of withdrawal, use of barrier methods of contraception, or contraceptive hormone use; those who used any of these methods were excluded from the study. These circumstances were uncommon: fewer than 10 couples were excluded for these reasons. We also reviewed any evidence of pregnancy, as suggested by prolonged postovulatory phase of a menstrual cycle. Follow-up beyond 12 months was accomplished mainly by telephone contact.
Throughout instruction, couples were advised that the CrMS can be used to achieve pregnancy as well as to avoid pregnancy. It was emphasized that having genital contact on a "day of fertility" has a high probability of resulting in pregnancy and that this would be considered a free and responsible choice to potentially achieve pregnancy. Instruction was given during the initial CrMS session and reinforced during follow-up visits that "there is no such thing as taking a chance with the ovulation method. You will know whether you are fertile or infertile on any given day." Importantly, the informed use of a fertile day for genital contact by a couple was not considered to be evidence of that couple necessarily "planning" or "intending" a pregnancy at a conscious level, but rather simply an acknowledgment that the couple had chosen to engage in "achieving-related behavior" (behavior that they knew to be likely to cause pregnancy). Assessment of these dynamics of use of the CrMS was accomplished by analyzing pregnancies in a fashion consistent with the teaching and use of the CrMS.
Whenever a pregnancy occurred, a detailed evaluation was done within the first 3 months of pregnancy. This evaluation comprised a review of the circumstances surrounding the pregnancy based on the daily CrMS record from the woman/couple, the routine teaching documentation of the CrMS practitioner, and a detailed interview with the woman/couple, usually in person, but sometimes by telephone. Based on this evaluation, all pregnancies were classified into 1 of the following categories: method-related pregnancies (occurring despite correct use of the CrMS to avoid pregnancy), pregnancies related to error in application of the CrMS by the woman/couple, pregnancies related to error in teaching the CrMS on the part of the teacher, pregnancies related to error on the part of both the woman/couple and the teacher, pregnancies caused by achieving-related behavior, or unresolved pregnancies (ie, insufficient information was available to classify the pregnancy into 1 of the first 5 categories). All categories were mutually exclusive (eg, if there was evidence of error by both the teacher and the user, the pregnancy was classified as teacher and woman/couple error and not as woman/couple error or teacher error). In all cases, the pregnancy classification was discussed with the woman/couple involved. Disagreement from the woman/couple regarding the classification was rare, but if there was disagreement, a second pregnancy evaluation was done by another individual. Evaluations for all pregnancies in this study were reviewed by one of us (M.P.H.).
We also grouped pregnancies together to estimate probabilities for the following clinically relevant outcomes: method-related pregnancy, avoiding-related pregnancy, and extended-use pregnancy. Method-related pregnancy, sometimes referred to as method failure, denotes pregnancies that occurred despite exactly correct use of a family planning method to avoid pregnancy, as objectively defined by the investigators.12 We used the term avoiding-related pregnancy to denote pregnancies that occur despite correct use of the CrMS to avoid pregnancy, as understood by the couple. Thus, avoiding-related pregnancy included pregnancies that occurred because of errors (by the couple or teacher) in addition to method-related pregnancies. We used different assumptions to distribute unresolved pregnancies among the other pregnancy categories to generate a range of estimates for the probabilities of method-related and avoiding-related pregnancy. For extended-use pregnancy,12 we included all pregnancies that occurred during a given time after beginning use of the CrMS; we also made further assumptions about pregnancies that may have occurred beyond our knowledge among those who were dropped from the study for reasons other than pregnancy.
This study involved review of the CrMS records as they are routinely kept by CrMS practitioners (teachers). No additional information was collected from couples, and all information was analyzed in a fashion that did not identify individual couples; hence, informed consent of the subjects was not sought for this study. The study protocol was reviewed and approved as exempt by the University of Utah Institutional Review Board for Human Subjects.
For each couple, the length of use of the CrMS prior to leaving the study (because of either pregnancy or other reasons) was calculated in ordinal months. The menstrual cycle was not used as a unit of measurement for this study. Those who were dropped from the study were then tallied by category and by time contributed to the study, as described by Hilgers,35 as adapted from Tietze and Lewit.36-37 From these tabulations, net probabilities of leaving the study were calculated for each category. A net life table gives the cumulative probability at a specified follow-up time (such as 1 year) of a subject exiting the study for each of a list of all possible reasons. Net life-table probabilities have been widely used to evaluate both contraceptives and NFP, but they are not ideal for comparing different studies, because discontinuation categories unrelated to pregnancy can influence pregnancy probabilities.11 Therefore, we also calculated gross life-table probabilities.38 Gross life-table probabilities give an estimate of the probability of pregnancy independent of any other competing category of leaving the study and hence can be used to compare pregnancy probabilities directly between different studies. However, gross probabilities have not been commonly reported in family planning studies. We reported our results in terms of probability of pregnancy rather than failure, since the term pregnancy is more precise for evaluating family planning methods.39
RESULTS
The demographic characteristics of the study sample are given in Table 1. In general, this was a well-educated, relatively affluent group of couples. About three fourths of women were of the Roman Catholic faith and most were either married or engaged. Almost all women (96%) had used the birth control pill at some point in their lives, and almost one third (29.1%) had used it immediately prior to beginning use of the CrMS. Slightly less than half of women (46.3%) were classified as having uncomplicated regular menstrual cycles (cycles lasting 21-38 days), without having any of the noted additional factors affecting fertility.
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Table 1. Demographic and Reproductive Characteristics of the Study Sample*
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A group of 701 clients contributed 6947.5 couple-months to this study over 18 months of follow-up. The net total pregnancy probabilities per 100 couples were 17.12 at 12 months and 21.26 at 18 months (Table 2). The net non–pregnancy-related probabilities of leaving the study were 27.39 at 12 months and 34.09 at 18 months (Table 3). Thus, the all-cause net probabilities of leaving this study (but not necessarily stopping use of the CrMS) were 44.51 at 12 months and 55.35 at 18 months.
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Table 2. Net and Gross Cumulative Probabilities of Pregnancy by Pregnancy Classification*
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Table 3. Net Cumulative Probability of Leaving the Study for Non–Pregnancy-Related Reasons by Reason for Leaving*
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Pregnancy probabilities by pregnancy classification and by month of study are presented in Table 2. Most pregnancies were caused by achieving-related behavior. A small proportion of pregnancies were caused by user error, teacher error, or a combination of user and teacher errors (net, <3.0 per 100 couples for all errors combined). A few pregnancies were unresolved because of insufficient data. There was only 1 method-related pregnancy. Table 2 presents both net and gross probabilities. While the same trends are observable for each measure, the gross probabilities, as expected, are somewhat higher.
Table 4 reports pregnancy probabilities at 12 months, stratified by the reproductive status of the woman at entry into the study, using the same reproductive status categories reported in Table 1. In this analysis, totally breastfeeding and breastfeeding/weaning were combined into the single category of breastfeeding. Because of small numbers of couples in some strata, the following categories were combined into a single category designated other: age 40 years or older, within 3 menstrual cycles after birth, not breastfeeding, and within 1 cycle after an abortion. The single method-related pregnancy in the study occurred in a woman who had stopped taking oral contraceptives in the past year. Overall, pregnancy probabilities were remarkably similar between reproductive categories. Pregnancy probabilities were notably higher in the breastfeeding category. The lowest total pregnancy probability at 12 months was in the uncomplicated regular cycles category; this reflected a lower probability of pregnancy because of achieving-related behavior in this category.
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Table 4. Net and Gross Cumulative Pregnancy Probabilities per 100 Couples at 1 Year by Reproductive Category at Study Entry and by Pregnancy Classification*
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Table 5 reports estimates of net probability for method-related pregnancy (pregnancy that occurred while the method was being used correctly), avoiding-related pregnancy (incorporating errors in use of the CrMS while it is still being used by the couple in a way in which they expect to avoid pregnancy), and extended-use pregnancy (incorporating all pregnancies and estimates of pregnancies for women who left the study for reasons not known to be related to pregnancy). For method-related and avoiding-related pregnancies, estimates are provided based on actual pregnancy probabilities, with differing assumptions regarding the resolution of unresolved pregnancies.35 For extended-use pregnancies, a low estimate included pregnancies known to have occurred in the study, and also assumed that there was an immediate probability of pregnancy of 0.05 per 100 couples among those who were dropped from the study for reasons not known to be related to pregnancy. Similarly, the high estimate for extended-use pregnancies assumes an immediate probability of pregnancy of 0.2 among those who were dropped from the study for reasons not known to be related to pregnancy. This percentage was chosen arbitrarily to exceed somewhat the overall 12-month pregnancy probability for this study. At 12 months, across the assumptions used for these estimates, the probability of method-related pregnancy ranged from 0.16 to 1.57; the probability of avoiding-related pregnancy ranged from 3.11 to 4.28, and the probability of extended-use pregnancy ranged from 18.47 to 22.51.
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Table 5. Estimated Probabilities of Method-Related, Avoiding-Related, and Extended-Use Pregnancy During Use of the Creighton Model Fertility Care System (CrMS)
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COMMENT
This study was designed to describe the real-world use of the CrMS rather than to focus solely on avoiding pregnancy. Thus, this study differs both from traditional contraceptive efficacy studies and from many efficacy studies of NFP by including all pregnancies (including those that may have been planned) in the classification of pregnancies and selection of subjects. This difference needs to be understood to compare this study with other studies of family planning. However, the different approach used in this study provides additional information that is highly relevant to the clinical use of the CrMS. When placed in appropriate context, the results of this study can be compared with research on other methods of NFP and contraception and can be used to advise couples about the option of the CrMS for family planning. The results of this study, in comparison with other studies, also raise important questions for future research.
The results of this study are similar to those found in previously published studies of the CrMS that have used the same classifications of pregnancy.28, 34, 40 These classifications are an inherent part of the way that the CrMS is taught and evaluated clinically. In our experience, these pregnancy classifications are readily accepted by nearly all CrMS users. The fact that each couple may choose during the study to change their use of NFP (to avoid or to achieve pregnancy)16 and that they may do so at any point without necessarily announcing it in advance is recognized and emphasized from the first CrMS session. We believe that these pregnancy classifications accurately reflect the decisions made by users of the CrMS as they choose whether to have genital contact at any given time.
A decision to have genital contact on a day of fertility was not defined as planning a pregnancy, but it was considered to be a decision to engage in achieving-related behavior. A typical example of this is reflected in the expression, "we weren't trying to get pregnant for another couple of months, but it didn't matter that much, so we thought we would take a chance." On the other hand, some of the pregnancies resulting from achieving-related behavior clearly represented conscious planning to get pregnant at that particular time. In this study, we did not differentiate between these 2 circumstances because we wished to reflect the use dynamics of this approach in the clinical teaching of the CrMS. This contrasts with the approach used in most contraceptive efficacy studies, which requires study participants to declare their intentions explicitly with regard to pregnancy at given intervals (eg, the next cycle, the next month, or the next year) and subsequently excludes from the study couples who report explicit intentions to achieve pregnancy. In this context, we reemphasize that no attempt was made in this study to select for couples who were committed to avoiding pregnancy for any period. Thus, the probabilities of achieving-related pregnancy (and total pregnancy probabilities) reported in this study include many pregnancies that most contraceptive efficacy studies (and many NFP studies) would have excluded on the basis that the couple had announced in advance that they intended to achieve pregnancy. One previous study of the CrMS evaluated all pregnancies (mostly within the first trimester) and reported that of the total pregnancies, couples reported that 56% were planned and 44% were unplanned.40 If we had followed the convention of other contraceptive research and excluded planned pregnancies, the reported overall pregnancy probabilities would have been lower. However, this would not have completely reflected the use dynamics of the CrMS, which can be used to achieve or avoid pregnancy.
In studies of other methods of NFP, pregnancies occurring among couples who have knowingly engaged in coitus on days defined as fertile by the NFP method under study have been classified by various investigators as pregnancies resulting from conscious departure from the rules,21 informed-choice pregnancies,7, 41 and pregnancies resulting from risk-taking.42 These terms, although conceptually distinct, may be comparable in practice with our term of pregnancy caused by achieving-related behavior, as long as planned pregnancies are not excluded. Some NFP studies exclude planned pregnancies, while others, like ours, do not. Most pregnancies that occur in all NFP studies result from genital contact during the fertile time, and variations in this category of pregnancy are the main determinant of variations in total pregnancy rates or probabilities in NFP studies.7, 43 The dynamics of planning pregnancy are not well understood.44 How intentions relate to sexual behavior among NFP users (as well as users of other family planning methods) remains an area for further study.
In this study, we have separately identified pregnancies resulting from an error in application of the CrMS by the couple, an error in teaching by the instructor, or both. The common mechanism underlying these pregnancies is that they result from the couple having genital contact at a time that they believed to be infertile, but that on review of their daily record was in fact defined as fertile by the correct application of the CrMS. Examples of such errors include not remembering (or not being taught) to observe for mucus discharge during the light days of the menstrual flow and having genital contact on a day of light flow in a cycle during which there was an early mucus buildup with early ovulation, not remembering (or not being taught) to consider nonmenstrual bleeding as a time of fertility, or misidentifying the peak day (the estimated day of ovulation). These categories of pregnancy (which ideally would have a probability of 0) constitute a key measure of the quality of teaching that is critical to proper use of NFP. Thus, the clinical significance of these categories of pregnancy is as a measure of the teaching process that is essential to optimal use of NFP (in this case the CrMS). Other NFP studies have reported teaching-related pregnancy probabilities or rates (combining these 3 categories) ranging from 0 to 12.2 per 100 couples.7
Despite our best efforts, we were unable to obtain sufficient information to classify a small number of pregnancies; these we reported as unresolved pregnancies. These pregnancies can be assumed to have the same general distribution as the pregnancies for which a classification is available, or they can be assumed to consist entirely of method-related pregnancies. This results in an estimated range of probabilities for method-related and avoiding-related pregnancies, as shown in Table 5.
The probabilities of avoiding-related pregnancy reported in Table 5 represent the probability of pregnancy that a couple can expect if they consistently use the CrMS to avoid pregnancy; in other words, if they have genital contact only on days they expect to be infertile according to the CrMS instructions. The pregnancies that occur in this circumstance are caused by an error that results in misclassifying a fertile time as infertile, either because of user error or because of inaccuracy intrinsic to interpreting the biological signs of fertility. We have further analyzed whether the user error was completely or partially caused by poor teaching or occurred in the setting of adequate teaching. This probability has high clinical relevance for users of NFP and is arguably a more relevant statistic to quote to a couple as best possible effectiveness (ie, lowest possible pregnancy probability) than the probability of method-related pregnancy, since it accounts for human error in teaching or learning. However, these estimates of the probability of avoiding-related pregnancy are not comparable with probabilities of use-related pregnancy in contraceptive efficacy studies, because in contraceptive efficacy studies (and some NFP studies), use-related pregnancy is defined in a way that includes some pregnancies caused by behaviors that the couple know are likely to result in pregnancy. We did not use the traditional definition of use-related pregnancy in this study because it does not coincide with how the CrMS is taught.
In theory, it would be possible for a contraceptive efficacy study to report a measure of the probability of avoiding-related pregnancy similar to what we have used in this study. For example, a study of a barrier method could differentiate pregnancies resulting from the complete lack of use of the barrier during a particular episode of sexual intercourse and pregnancies resulting from improper use of a barrier because of misunderstanding how to use it. To our knowledge, such a study does not exist for contraceptive methods, despite suggestions of some researchers that such distinctions would be clinically meaningful.44
How then can one best compare the pregnancy probabilities in this study with those of other family planning methods? Some might directly compare the total pregnancy probability in our study with the total pregnancy rate or probability from studies of other NFP or contraceptive methods. For example, the probability of pregnancy during the first year of typical use (or probability of use-related pregnancy) is reported for barrier methods to be from 12% to 22%,45-46 for spermicides from 11% to 31%,17 and for oral contraceptives from 3% to 7%.45-46 However, in this type of comparison, the CrMS net total pregnancy probability of 17.12 per 100 couples is artificially high compared with pregnancy probabilities of other methods because the CrMS probability includes pregnancies that the other methods would have excluded as planned pregnancies.
Conceptually, the best direct comparison of the pregnancy probabilities of the CrMS in this study with other methods of family planning would probably employ the concept of extended-use pregnancy. The concept of extended-use pregnancy was developed to include compliance and longer-term acceptability of family planning methods as an essential component of their ultimate effect to avoid pregnancy. Extended-use pregnancies include all pregnancies that occur during the study period, regardless of whether the method under study was still in use to avoid pregnancy.10, 12, 38, 47 For example, the probability of an extended-use pregnancy over 1 year for an oral contraceptive would take into account all pregnancies that occur within the year after beginning use of the oral contraceptive, regardless of consistency of use, including even those that occurred in women who discontinued use of the oral contraceptive altogether during that year (and may or may not have resumed its use during the same year). This would be comparable with including pregnancies among NFP users who begin having genital contact during the fertile time as well as among those who abandon the use of NFP altogether. Probabilities for extended-use pregnancy at 12 months by net life-table analysis have been reported for the intrauterine device (range, 4.8-16.3 per 100 couples) and for oral contraceptives (range, 8.4-39.5).48-50 Probabilities of extended-use pregnancy for reversible methods of contraception are higher than the corresponding probabilities of use-related pregnancy because some women discontinue use of the contraceptive altogether. In this study, we estimated a net probability of extended-use pregnancy of the CrMS at 12 months of 18.47 to 22.51 per 100 couples, based on varying assumptions of what percentage of persons who left the study for non–pregnancy-related reasons ultimately ended up pregnant. However, these CrMS estimates include pregnancies that the contraceptive studies would have excluded as having been planned. Additionally, although the concept of extended-use pregnancy has a well-respected history in the medical and family planning literature, it has fallen into disuse in recent years and has been reported only occasionally in recent studies, although it is not always explicitly referred to as such.48 Despite this, we advocate its continued use because it is a highly relevant measure of pregnancy probability for a given method of family planning from both clinical and demographic perspectives, particularly if it is defined in such a way as to include planned pregnancies, as we have done in this study.
Taking these factors into consideration, it seems likely that the pregnancy probabilities of the CrMS that would be most comparable with the probabilities or rates of use-related pregnancy reported for other methods of family planning are between the estimates for avoiding-related pregnancy and extended-use pregnancy given in Table 5.
As recommended in previous studies,12, 51 we have reported gross and net life-table probabilities for pregnancy. Since gross probabilities adjust for subjects who left the study for non–pregnancy-related reasons (eg, moved or lost to follow-up), they are somewhat higher than net probabilities. Gross probabilities are more comparable between studies, but few family planning studies have reported gross probabilities. While we have emphasized net life-table probabilities in this report because of their standard use, we have reported gross probabilities for comparisons with other studies that may also make these available. Gross probabilities from this study cannot be directly compared with net probabilities or Pearl rates from other studies.
The net life-table probabilities in this study are based on a common denominator of all study subjects using a method that was developed by leading contraceptive researchers and is still widely used. However, more recent literature has suggested that it is more accurate to divide subjects or cycles of use into 2 groups—perfect users (or perfect-use cycles) and imperfect users (or imperfect-use cycles)—and to calculate separate perfect-use and imperfect-use pregnancy probabilities based on these separate denominators.45, 51 Since the CrMS is taught and used in a conceptual framework that emphasizes that couples have the freedom, the responsibility, and the capability to make choices about their reproductive capacity at all times, we suggest that an appropriate approach for the CrMS would be to evaluate pregnancy probabilities for months when couples knowingly had genital contact during times of fertility (which could be called an adjusted achieving-related pregnancy probability), and pregnancy probabilities for months when couples had genital contact only during times they knew to be infertile (which could be called an adjusted avoiding-related pregnancy probability). Unfortunately, we do not have data on whether genital contact occurred at the fertile time for every couple-month in our study (we have such data only for months when pregnancy occurred), and thus we cannot address this issue definitively. However, the available evidence suggests that couples who become pregnant during use of the CrMS do so within very few cycles of genital contact during the fertile time (76% within 1 cycle in 1 published study).52 Therefore, the adjusted achieving-related pregnancy probability would likely be very high (very much higher than the achieving-related pregnancy probability reported in this study), and the adjusted avoiding-related pregnancy probability would likely be only slightly higher than the probabilities of avoiding-related pregnancy reported in Table 5. This reasoning is consistent with an analysis of a multinational study of the ovulation method that found that genital contact only on days of infertility within the ovulation method is highly unlikely to result in pregnancy, whereas genital contact on days of fertility is highly likely to result in pregnancy.22 However, further research is needed to define the exact probabilities for adjusted achieving-related and adjusted avoiding-related pregnancy.
A related issue has to do with the first cycle of observation, during which couples are advised to abstain from all genital contact to facilitate their learning of the CrMS (to avoid confusion from seminal residue). Some have suggested that this first cycle should be excluded from analysis of pregnancy probabilities, since no exposure to the possibility of pregnancy was to have occurred. However, not all couples followed this instruction, and there were some pregnancies during the first month of use. We have followed the approach of other researchers studying the ovulation method by including the first cycle in our analysis.22 Furthermore, alternate analysis excluding the first cycle (and attributing pregnancies to the second cycle) results in nearly identical 12-month life-table pregnancy probabilities (data not shown).
It is possible that some couples with undiscovered reduced fertility may have been included in this study, which would lower the probability of pregnancy. Some studies of NFP or contraception have dealt with this problem by including only couples of proven fertility (ie, those who have previously had children).30 In this study, we chose to maximize generalizability by including all couples except those with a known history of infertility. It has been suggested that couples who choose to use NFP or who continue its use have, on average, lower fertility than couples who use other methods. We know of no evidence to support this suggestion. Couples who choose to use NFP seem to be motivated by concerns about health, by side effects from other methods, or by moral beliefs,53-55 reasons that are unlikely to be associated with reduced fertility.
Most studies of NFP or contraceptive efficacy have excluded women who have recently taken oral contraceptives, who are breastfeeding, or who have other conditions that might be associated with a temporary reduction in fertility. However, it is important to know how a method performs among couples with these various circumstances. Therefore, this study included essentially all new users of the CrMS. This allows for much wider generalizability of these results than has been possible with previous studies of NFP, most of which have included only women with regular, uncomplicated cycles. To provide relevant comparisons with such studies, we have reported separate pregnancy probabilities for subjects with uncomplicated regular cycles, as well as for long cycles, discontinuing oral contraceptives, breastfeeding, and other circumstances (Table 4). The total pregnancy probability was lowest for couples with uncomplicated regular cycles (net probability of 13.98 per 100 couples at 12 ordinal months for couples with uncomplicated regular cycles vs 17.12 for the entire study). This does not mean that the CrMS is intrinsically more effective for those with uncomplicated regular cycles, because the probability of method-related pregnancy is low for all of the groups, and the differences are mainly caused by achieving-related behavior. It seems that relatively new users of the CrMS who have uncomplicated, regular cycles are less likely to engage in achieving-related behavior (genital contact on days known by the couple to be fertile). From our clinical experience, we feel that this might be because new users of the CrMS rely to some extent on calendar rhythm despite all the teaching of the CrMS to the contrary. Couples who have irregular cycles and experience a time of fertility (as defined by mucus discharge changes) that differs in calendar timing from what they expect may be less likely to believe that this time is truly fertile as indicated by the CrMS and thus may be more likely to take a chance, at least initially. However, this explanation is based on anecdotal reports and requires further research.
While fertility usually returns rapidly after discontinuing use of oral contraceptives,56 some cycle abnormalities exist in this circumstance that may make it more difficult for a couple to begin use of NFP.57 These data indicate that couples who discontinue use of oral contraceptives can use the CrMS effectively. Since a substantial proportion of those beginning use of the CrMS are discontinuing use of oral contraceptives, this is an important finding.
Similarly, a high proportion of couples who are beginning use of the CrMS are breastfeeding, a situation for which relatively few data exist on the effectiveness of NFP. One notable exception is the lactational amenorrhea method, a method of NFP based entirely on breastfeeding that has been shown to have a pregnancy probability of less than 2% in the first 6 months postpartum, as long as breastfeeding is total without supplementation and menses has not returned.58-59 However, most women in the United States do not breastfeed in a way that meets the criteria for the lactational amenorrhea method, and pregnancy probabilities rise rapidly once weaning has been initiated or menses has returned.60 Even couples who do use the lactational amenorrhea method enter a period of transition when other options for family planning are needed.61 Unfortunately, we do not have data to report what percentage of breastfeeding couple-months in this study would have also met the criteria for the lactational amenorrhea method, so we cannot estimate an independent effect of the CrMS vs the intrinsic lower fertility of the total breastfeeding state. In addition, we have reliable information on breastfeeding status only at entry to the study; we do not know how long these women continued to breastfeed, when they weaned, or whether they were still breastfeeding at the time that they became pregnant.
Since breastfeeding reduces fertility, the inclusion of breastfeeding couples in this study theoretically could have lowered overall pregnancy probabilities, similar to the effect found in one previous study of the ovulation method in postpartum women that found a lower overall pregnancy rate among women who were breastfeeding.62 However, the opposite occurred: total pregnancy probabilities were higher among the breastfeeding couples (23.8 per 100 couples at 1 year for breastfeeders vs 17.1 for all couples). While counterintuitive, this finding does have precedent: Labbok et al60 have reported higher rates of pregnancy among women using the ovulation method who were breastfeeding compared with women who were not, and this occurred entirely after the criteria for the lactational amenorrhea method were no longer met. They also presented data that suggested that this was not caused by increased frequency of intercourse during the fertile time. In contrast, our study indicated that the increased total pregnancy probability during use of CrMS primarily reflected a higher probability of pregnancy because of achieving-related behavior. Overall, this study suggests that the CrMS can be used effectively by breastfeeding couples, but that breastfeeding couples may be more likely than others to get pregnant as a result of genital contact on days known by the couple to be fertile. Further research on the use of the CrMS in postpartum couples (both breastfeeding and not) will be needed to clarify the reasons for the higher probability of pregnancy among breastfeeding couples. It will be particularly important to track the timing of supplementation, weaning, and return of menses in relation to pregnancy probabilities among women who are breastfeeding.
While the probability of leaving this study for reasons other than pregnancy was 27.39 per 100 couples at 1 year, the probability of loss to follow-up was 12.4. Leaving the study did not necessarily mean discontinuation of use of the CrMS (Table 3). At 18 months, the probability of loss to follow-up rose to 17.4, probably because the follow-up mechanisms are less formalized after 1 year of use of the CrMS. Probabilities or rates of loss to follow-up (generally at 1 year) in other studies have been reported from 0 to 59.5 for spermicides, from 0 to 43.8 for barrier methods, from 0.3 to 32.0 for oral contraceptives, and from 0.3 to 33.9 for other NFP methods.10
Women (and couples) are often not satisfied with the generally available methods of family planning.63 There is evidence that more women are potentially interested in NFP methods than currently use them.64-65 Population-based studies in Germany and the United States suggest that up to 25% of women of reproductive potential may be interested in using modern NFP methods to avoid pregnancy and up to 33% may be interested in using them to achieve pregnancy. Most of this interest seems to be motivated by health concerns rather than religious concerns.55, 66 In light of this latent and largely unrecognized demand, health care professionals need to be aware of the availability and viability of modern methods of NFP. We feel that the CrMS is particularly well suited to meet this need because of its highly trained instructors and standardized teaching methods.3, 28, 34
Couples using the CrMS in this study were of relatively high socioeconomic and educational status. However, there is no evidence that education is a prerequisite for the successful use of NFP. In a study of the Billings Ovulation Method conducted by the World Health Organization,30 illiterate women in El Salvador learned to understand their fertility cycles just as rapidly and easily as educated women in New Zealand and Ireland. Natural family planning has been used successfully in a variety of Third World countries.6, 67-68 Furthermore, studies of potential interest in NFP in developed countries have shown no association with socioeconomic status or education.55, 66 We believe that the socioeconomic and educational status of couples in this study primarily reflects knowledge of and access to the CrMS; thus, efforts to make the CrMS available to a more diverse population are warranted. We do emphasize that use of NFP requires the cooperation of both partners and that NFP is not suitable for those who are not in monogamous relationships.
Based on the results of this study, we suggest that several points be emphasized in counseling couples about the CrMS: First, if you use the CrMS perfectly to avoid pregnancy, the probability of pregnancy within the first year is less than 1%. However, accounting for errors (which could be made by the users and/or the teacher), the probability of pregnancy within the first year during consistent use to avoid pregnancy is about 3% to 4%. Second, those of normal fertility who have genital contact on any day defined as fertile by the CrMS will very likely become pregnant (one study suggests more than a 50% probability of pregnancy within the first cycle).52 Third, in our study, the probability that couples who started use of the CrMS would be pregnant in 1 year was about 17%. Most of those pregnancies were a result of the couple having genital contact on a day they knew to be fertile, for many reasons, including taking a chance or planning a pregnancy. Fourth, women who have regular cycles, who have irregular cycles, who are breastfeeding, or who are discontinuing use of oral contraceptive pills can use the CrMS successfully. Fifth, couples in which the woman is breastfeeding may have a higher chance of pregnancy with the CrMS if they have genital contact during a time of fertility. Finally, successful use requires learning the CrMS from a qualified Creighton Model instructor. Given time constraints, this will not ordinarily be the physician or provider, but rather a trained instructor to whom the provider can refer or who may provide instructional services within the same office.
CONCLUSIONS
This study supports the potential effectiveness of the CrMS in a broad spectrum of reproductive categories in a US population of users. The strengths of this study included the inclusion of couples in all reproductive situations, the comparison of pregnancy probabilities according to reproductive status of the subject at entry, an analytical approach that reflected how the CrMS was actually taught and used, reporting both gross and net probabilities, results that provide information highly relevant to the clinical use of the CrMS, and a reasonable rate of loss to follow-up. The weaknesses of this study included the lack of information about how many couples who had genital contact during the fertile time did not conceive, a lack of information about the timing of weaning and the return of menses among the breastfeeding subjects, and the inability to compare pregnancy probabilities directly with those of many contraceptive studies because of the inclusion of pregnancies that would have been excluded by those studies as planned pregnancies. Despite the weaknesses of this study, the results provide a strong basis for counseling couples about this important option for family planning, as described above. This research also suggests many pressing issues for further inquiry, including the relationships among achieving-related behavior (sexual behavior known to be likely to cause pregnancy), taking chances, and planning pregnancy, both among NFP users and among users of other methods of family planning. Another important area for further research is the relationships among the infertility of lactational amenorrhea during total breastfeeding, the return of fertility during weaning and/or resumption of menses, and the use of the CrMS in these circumstances.
AUTHOR INFORMATION
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Accepted for publication September 24, 1998.
We thank Charles E. Howard, MFA, CNFPP, for his assistance with the collection and organization of data for this study; Ken R. Smith, PhD, for his help with statistical analysis; and the NFP practitioners for teaching the couples who were subjects of this study.
| Contact Information
Creighton Model teachers (practitioners) are available in most states and metropolitan areas. Some are associated with health care institutions, such as hospitals or private physician practices, some are associated with Catholic diocesan offices, and some work independently. To locate a Creighton Model teacher in your area, contact the American Academy of Natural Family Planning at 775-827-2500, online at http://www.aanfp.org, or through the Pope Paul VI Institute for the Study of Human Reproduction at 402-390-6600, online at http://www.popepaulvi.com. Creighton Model teachers are trained in many centers in the United States; a current listing of Creighton Model teacher training programs can be obtained by contacting the American Academy of Natural Family Planning.
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Corresponding author: Joseph B. Stanford, MD, MSPH, Department of Family and Preventive Medicine, University of Utah, 50 N Medical Dr, Salt Lake City, UT 84132 (e-mail: jstanford{at}dfpm.utah.edu).
From Fertility Care Services, Covenant Health System, Lubbock, Tex (Ms Howard); and the Department of Family and Preventive Medicine, University of Utah, Salt Lake City (Dr Stanford). Ms Howard was formerly with the Department of Natural Family Planning, St Joseph's Hospital, Houston, Tex.
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