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Vulvar Mucus Observations and the Probability of Pregnancy

From the Health Research Center, Department of Family and Preventive Medicine, and Department of Family and Consumer Studies, University of Utah, Salt Lake City, Utah; and Biostatistics Branch, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina.

Address reprint requests to: Joseph B. Stanford, MD, MSPH, 375 Chipeta Way, Suite A, Salt Lake City, UT 84108; E-mail: jstanford{at}dfpm.utah.edu.

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
OBJECTIVE: To assess the day-specific and cycle-specific probabilities of conception leading to clinical pregnancy, in relation to the timing of intercourse and vulvar mucus observations.

METHODS: This was a retrospective cohort study of women beginning use of the Creighton Model Fertility Care System in Missouri, Nebraska, Kansas, and California. Data were abstracted from Creighton Model Fertility Care System records, including women’s daily standardized vulvar observations of cervical mucus discharge, days of intercourse, and clinically evident pregnancy (conception). Established statistical models were used to estimate day-specific probabilities of conception.

RESULTS: Data were analyzed from 1681 cycles with 81 conceptions from 309 normally fertile couples (initially seeking to avoid pregnancy) and from 373 cycles with 30 conceptions from 117 subfertile couples (who were initially trying to achieve pregnancy). The highest probability of pregnancy occurred on the peak day of vulvar mucus observation (.38 for normally fertile couples and .14 for subfertile couples). The probability of pregnancy was greater than .05 for normally fertile couples from 3 days before to 2 days after the peak, and for subfertile couples from 1 day before to 1 day after the peak. The cycle-specific probability of conception correlated with the quality of mucus discharge in normally fertile couples but not in subfertile couples.

CONCLUSION: Standardized vulvar observations of vaginal mucus discharge identify the days with the greatest likelihood of conception from intercourse in normal fertility and subfertility and provide an indicator of the overall potential for conception in a given menstrual cycle in normal fertility.

Estimating the probability of pregnancy from intercourse during a given day of the human menstrual cycle is of interest to couples who are seeking to achieve pregnancy, to clinicians who advise such couples, and to scientists studying fertility. To date, three data sets have been used to estimate the probabilities of pregnancy for given days relative to an estimated ovulation day identified by basal body temperature or urinary hormones.^1–6 These analyses have suggested that virtually all conceptions are attributable to intercourse during a window from 5 days before ovulation to the day of ovulation itself.^4,6 Couples with subfertility were excluded from these studies.

Estrogenic cervical mucus is necessary for fertility because of its role in sperm survival and transport through the cervix.^7–9 Women’s observations of changes in their vaginal discharge correlate very closely with the changes in the biophysical parameters of the mucus secreted by the cervix¹⁰ and are the basis of well-established clinical methods for identifying days during the menstrual cycle during which intercourse may result in pregnancy. These methods include the Billings Ovulation Method^11,12 and the Creighton Model Fertility Care System.^13,14 With these methods, pregnancy is highly unlikely when intercourse occurs on days identified as infertile^14–18 and very likely on days identified as fertile.^19,20 However, the probability of pregnancy from intercourse on days identified as fertile by the Creighton Model Fertility Care System or the Ovulation Method has not been previously quantified.

The objectives of this study were 1) to estimate the day-specific probabilities of conception in the menstrual cycle based on the daily standardized vulvar observation of mucus discharge as a clinical marker of ovulation and 2) to assess the cycle-specific probability of conception associated with the quality of mucus discharge.

For this study, conception means clinically identified pregnancy. We studied new users of the Creighton Model Fertility Care System, including normally fertile and subfertile couples.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Ours was a retrospective cohort study using data extracted from existing standard teacher and user records of the Creighton Model Fertility Care System. All new users of the Creighton Model Fertility Care System from six different Creighton Model Fertility Care System centers in four different cities (in Missouri, Nebraska, Kansas, and California) for the years 1990–1996 were considered potentially eligible for this study. A detailed description of these new users has been published previously.²¹

Based on computer simulations, our goal at the outset of the study was to obtain data from 500 couples, with approximately 100 of those couples being subfertile. We used the stated intention for learning the Creighton Model Fertility Care System at the time of entry to develop two distinct cohorts, as described below.

We selected normally fertile couples from those learning the Creighton Model Fertility Care System with an initial intention of avoiding pregnancy or of monitoring their fertility without specific reproductive intentions. We applied the following selection criteria to this group: age at entry between 18 and 39 years, sexually active within two menstrual cycles of entry, not pregnant at entry, not breast-feeding, no history of disease or surgery that might impair fertility, records available for review, and completion of four or more follow-up instructional visits. Most couples learning the Creighton Model Fertility Care System complete at least four follow-up visits, usually in the first 2 months of use.²¹ Couples of normal fertility who are learning the Creighton Model Fertility Care System are instructed that whenever they choose to have intercourse during days of fertility, pregnancy is the expected outcome.¹⁴

We selected subfertile couples from those who indicated at entry to Creighton Model Fertility Care System instruction that they were trying to achieve pregnancy. Most of these couples (71%) had previous difficulty in achieving pregnancy. We applied the following selection criteria to this group: woman’s age at entry between 18 and 39 years, sexually active within two menstrual cycles of entry, not pregnant at entry, not breast-feeding, and records available for review. For this group, we had no exclusion for number of follow-up visits, to include pregnancies that might happen in the first 2 months. There was no exclusion based on disease or surgery.

Data abstraction commenced when the woman first started charting her daily vaginal discharge, after she attended an introductory teaching session for the Creighton Model Fertility Care System. Each couple continued contributing data to the study until any one of the following conditions: 1) The woman became pregnant, 2) no further Creighton Model Fertility Care System charting was available, 3) the woman began the use of hormonal therapy, or 4) the couple completed 1 year of charting. An additional exit condition for the couples with apparently normal fertility was any reproductive surgery or procedure (such as cryotherapy of the cervix).

Four standard Creighton Model Fertility Care System forms were used: 1) the general intake form filled out when a couple first starts Creighton Model Fertility Care System instruction, which includes demographic information and medical histories from the woman and her male partner (this form was obtained for all potential participants in the study and used for subsequent eligibility screening); 2) the follow-up form, a detailed checklist of items assessed by the Creighton Model Fertility Care System teacher during follow-up instruction (including completeness of recording acts of intercourse and use of barriers or withdrawal); 3) the Creighton Model Fertility Care System daily chart, upon which a couple charts the woman’s daily vaginal discharge and whether coitus occurred each day (photocopied by the teacher during follow-up instruction); and 4) the Creighton Model Fertility Care System pregnancy evaluation form, filled out by the Creighton Model Fertility Care System teacher, ordinarily within a few weeks after a pregnancy is clinically identified.

Charts were photostatically copied by each of the Creighton Model Fertility Care System centers, with identifiers removed, and sent to the University of Utah. Research assistants who were formally trained in the Creighton Model Fertility Care System abstracted data from the charts based on protocols developed by the investigators, including the items described in the previous paragraph. The peak day for each menstrual cycle was identified independently by two research assistants and by the principal investigator (JBS).

For the purposes of this analysis, menstrual cycles were excluded if any of the following was present: concurrent use of hormonal medication (including oral contraceptives) or any other medication that might affect fertility or mucus observations (such as clomiphene), incomplete data for mucus observations, incomplete data for acts of intercourse in the cycle, any use of barriers or withdrawal during the cycle, and inability of research personnel to reach consensus on the peak day (3.7% of cycles).

To incorporate data from menstrual cycles that have multiple days with intercourse during the potentially fertile portion of the cycle, statistical models need to be used to estimate the day-specific pregnancy probabilities.^1,4–6 The most commonly used model was derived by Schwartz et al (1980)³ under the assumption that batches of sperm introduced into the reproductive tract on different days mingle and then compete independently in attempting to fertilize the ovum. By assuming that all couples have equivalent probabilities of conception in a menstrual cycle given equivalent timing of intercourse, the Schwartz et al model can produce biased estimates. Our analysis is instead based on a recently proposed generalization of the Schwartz model that accounts for observed and unobserved couple- and cycle-specific factors predicting a couple’s reproductive capacity.²² We incorporate these factors through a "cycle viability" term, which has a multiplicative effect on the day-specific pregnancy probabilities. By requiring the day-specific probabilities to increase up to the cycle maximum on the unknown most fertile day of the cycle and decrease thereafter, this model produces more efficient estimates than earlier unconstrained models. This model specifically accounts for the fact that multiple menstrual cycles from the same woman are not independent events.²³

The standardized daily vaginal discharge recording system of the Creighton Model Fertility Care System is based on stretch, color, and sensation of the discharge.¹⁰ This recording system can be converted to a daily numerical score ranging from 0 to 16.²⁴ An average of the daily score from the peak day and the 5 days preceding yields a summary mucus cycle score for each menstrual cycle. We used this summary mucus cycle score as a covariate for predicting cycle viability in our models, along with age and recent use of oral contraceptives.

The study protocol was approved by the University of Utah Institutional Review Board as exempt (not requiring subject consent) because it involved the use of existing clinical data with all unique subject identifiers removed.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The demographic characteristics and reproductive history of the initially screened sample have been reported in detail elsewhere.²¹ (One Creighton Model Fertility Care System center that initially contributed screening data withdrew from the study before contributing final data.) Of 1278 new users of the Creighton Model Fertility Care System at the six centers, 184 were excluded because they were breast-feeding or pregnant, 72 because they were younger than 18 years or older than 39, and six because their intention status at entry was unknown. This left 816 couples who were not seeking pregnancy at entry and 200 subfertile couples who were seeking pregnancy at entry. Of the 816 couples not seeking pregnancy at entry, 94 were excluded because of a history of gynecologic surgery that could affect fertility or mucus observations, 15 were excluded because of past use of medroxyprogesterone acetate (Depo-Provera; Pharmacia & Upjohn Co., Kalamazoo, MI) or the intrauterine device, and 139 were excluded because they completed less than four follow-up Creighton Model Fertility Care System instructional visits, leaving 568 couples. Of these, 259 charts (46%) could not be retrieved, leaving 309 fertile couples with charts available. Of the 200 subfertile couples, 83 charts (43%) could not be retrieved, leaving 117 couples with charts available.

Table 1 gives the demographic and reproductive characteristics of the 309 normally fertile couples and 117 subfertile couples included in the analysis. The sample was predominantly white (93%), married (71%; an additional 19% were engaged), educated (65% college graduates), Catholic (71%), and nulliparous (67%). There were differences between normally fertile and subfertile couples in the proportion from different Creighton Model Teaching Centers, marital status (married much more likely to be subfertile), and religion (Catholic women more likely to be normally fertile). Recent users of oral contraceptives were more likely to be in the normally fertile group. Subfertile couples were more predominant among those with a history of pregnancy and spontaneous abortion and those who had previously used a natural method of family planning, most commonly basal body temperature (which may have been used to try to achieve pregnancy). Subfertile couples were also older than normally fertile couples (both men and women) and on average entered Creighton Model Fertility Care System instruction about 1 year later.

From the 309 normally fertile couples we initially had 2142 menstrual cycles recorded. After exclusion of cycles with use of problematic medications, use of barriers or withdrawal, or incomplete recording of mucus observations or intercourse, 1681 cycles were eligible for analysis, among which were 81 pregnancies. Of the 1681 cycles, 550 had intercourse some time in the interval from 6 days before to 3 days after the peak day, and all 81 pregnancies were within this subgroup of cycles. From the 117 subfertile couples we initially had 600 menstrual cycles recorded. After exclusion of cycles for the same reasons, 373 were eligible for analysis, among which were 30 pregnancies. Of the 373 cycles, 260 had intercourse sometime in the interval from 6 days before to 3 days after the peak day, and all 30 pregnancies were within this subgroup of cycles. For both normally fertile couples and subfertile couples the median length of menstrual cycles entered into the study was 29 days, with 80% of menstrual cycle lengths falling between 25 and 38 days, inclusive. For both groups combined, the total numbers of acts of intercourse that occurred in the analyzed cycles in the window from 6 days before to 4 days after the peak day were as follows: 296 acts of intercourse on day -6, 249 on day -5, 214 on day -4, 200 on day -3, 183 on day -2, 178 on day -1, 179 on day 0 (the peak day), 164 on day +1, 146 on day +2, and 147 on day +3.

The highest probability of conception was on the mucus peak day (day 0): .38 for normally fertile couples and .14 for subfertile couples (Figure 1). The probability of pregnancy (point estimate) was greater than .05 for normally fertile couples from day -3 to day +2 and for subfertile couples from day -1 to day +1. Similarly, the probability of pregnancy (point estimate) was greater than .01 for normally fertile couples from day -5 to day +4 and for subfertile couples from day -4 to day +3.

View larger version (20K): [in this window] [in a new window]   Figure 1. Probability of pregnancy from intercourse on a single day in the menstrual cycle relative to the peak mucus day, plotted separately for normally fertile couples and subfertile couples. The dotted and dashed lines represent 90% confidence intervals for each group. Stanford. Probability of Conception. Obstet Gynecol 2003.

View larger version (19K): [in this window] [in a new window]   Figure 2. Correlation between mucus cycle score and the probability of pregnancy from intercourse on the peak mucus day of the menstrual cycle for cycles from normally fertile couples and subfertile couples. Finely dotted lines represent the 90% confidence intervals for the normally fertile couples, and the longer segmented lines the 90% confidence intervals for the subfertile groups. Stanford. Probability of Conception. Obstet Gynecol 2003.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

The maximum probability of conception in our study in couples of apparently normal fertility (from intercourse on the peak day) is .38, which is slightly lower than estimates from the British study and slightly higher than results from the North Carolina and European studies.^4,6 The duration of the fertile window identified in our study for normally fertile couples (6 days for probabilities greater than .05) is comparable to that found in previous studies,^4–6 whereas the duration of the fertile window is less for subfertile couples (3 days for probabilities greater than .05).

This study differs significantly from previous analyses of the day-specific probability of conception in suggesting that the highest probability of conception occurs on the peak mucus day (our estimated day of ovulation) rather than 1 or 2 days previous.^4,6 All studies of the fertile window have used imperfect markers of ovulation that have some variability around the actual time of ovulation. Studies have shown that the peak day is distributed within a range of plus or minus 3 days relative to ovulation (measured hormonally or by ultrasound) but that it is more likely to occur before ovulation than after. In contrast, the basal body temperature shift is more likely to occur after ovulation.^{11,26,28–30} This might in part account for the apparent discrepancy with prior analyses studies based on basal body temperature.^4,6 In addition, the peak mucus day probably identifies the day when sperm are most likely to survive in vivo.^9,10 The lack of an independent marker of ovulation is a weakness of this study; studies that compare the peak day to another marker of ovulation are needed to further clarify the relationship of the day of maximum fertility, the mucus peak day, and the day of ovulation.

Other studies have suggested that recent use of oral contraceptives may reduce the probability of conception^31,32 and that older age of the woman is associated with a lower probability of conception.^6,23,33 However, these variables were not significant in our study. We might have found a significant effect for age if we had been able to include more women of younger ages because there was a trend to decreasing probability of pregnancy with older age. Similarly, there were relatively few cycles soon after discontinuing oral contraceptives.

Our study shows that the mucus cycle score from the days immediately preceding ovulation is positively correlated with the cycle-specific probability of conception among couples of apparently normal fertility. This cycle effect is consistent with a previous analysis that found a day-specific effect.³⁴ Mucus discharge serves as a marker for the likelihood of sperm survival, but it also depends on the estrogen levels during the follicular phase and therefore may reflect the hormonal status of the menstrual cycle, which in turn may be related to the viability of the ovum.³⁵ However, there was no relation for subfertile couples, for whom other factors that impair fertility (such as anatomic factors or male factors) are likely to have higher prevalence and more influence.

Our findings must be interpreted in light of some uncertainty in the actual fertility status of the couples in the study because the primary classification of this is based on the intention of couples at entry to the study. The majority of the couples of apparently normal fertility were nulliparous, and all but two of the unmarried couples in the study were in the normally fertile group. Therefore, this group probably included some couples with unrecognized subfertility who had had no previous opportunity to test their fertility. This is similar to the previous studies of day-specific probabilities of conception.^2,5,6 On the other hand, it is possible that some couples learning the Creighton Model Fertility Care System with the initial intent to achieve pregnancy in fact had normal fertility. However, at a bare minimum, two thirds of the subfertile couples had well-documented infertility, and most had experienced difficulty in trying to achieve pregnancy.

Because of the retrospective nature of this study, we were unable to locate over 40% of the charts from potentially eligible couples. We explored whether this may have influenced the reproductive potential of the sample. As compared by ² analysis, the availability of charts was not associated with the following characteristics: fertility status, woman’s or man’s age, education, income, religion, parity, or history of spontaneous or elective abortion. There was a significant association with marital status (71% of those in the study were married, as opposed to 49% of those whose charts could not be located [P < .001]). Most importantly, it is probable that more charts were available for those couples who became pregnant, which might bias the probabilities of conception upwards (if the unavailable charts included additional cycles with intercourse in the fertile window but without conception). However, it is reassuring that the overall estimates for the normally fertile group are in the same range as estimates from previous studies.^4,6

Another potential for bias is the exclusion of couples in the normally fertile group who completed fewer than four follow-up visits. If there were many conceptions that were excluded during this first 2 months, this could bias the probabilities of pregnancy downwards among the normally fertile couples. Given that couples in this group were avoiding pregnancy at study entry, we believe that this bias should be small.

Our study has implications for research in fertility. Studies that seek to identify day-specific or cycle-specific probabilities of conception in couples of normal fertility may be incomplete without an assessment of mucus discharge quality. The Creighton Model Fertility Care System has a standardized vaginal discharge recording system and mucus score and standardized instruction procedures for patients that are well suited for this type of assessment.^{13,14,21,36,37} Because vulvar mucus observations identify the days when intercourse is most likely to result in pregnancy among subfertile couples, further study of the clinical application of vulvar mucus observation for infertility is warranted. In the meantime, the present study confirms that the Creighton Model Fertility Care System can be used by both normally fertile and subfertile couples wishing to optimally time intercourse to achieve pregnancy.³⁸

	Footnotes

 
Primary funding for this study was provided by the Robert Wood Johnson Foundation through the Generalist Physician Faculty Scholars Program (JBS). Additional funding was provided by the Health Studies Fund, Department of Family and Preventive Medicine, University of Utah.

The authors thank Becky Crockett, Mary Bishop Stone, and Julie Fryer for research assistance. Thomas W. Hilgers, MD, and James Trussell, PhD, served as consultants to this study.

doi:10.1016/S0029-7844(03)00358-2

Received July 12, 2002. Received in revised form January 10, 2003. Accepted January 23, 2003.

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