Study Design

The North Carolina Early Pregnancy Study was a prospective cohort study conducted to determine the risk of early loss of pregnancy among healthy women.5,6 From 1982 through 1985, we recruited from the local community 221 women who were planning to become pregnant. Women were excluded if they had a serious chronic illness or if they or their partners had a history of fertility problems. Ninety-six percent of the participants were white, and 71 percent were college-educated. Eighty percent were between the ages of 26 and 35, and one third were nulliparous.6 All the women who were enrolled gave their written informed consent, and our research protocol was reviewed and approved by the institutional review board of the National Institute of Environmental Health Sciences.

Details of the protocol have been published previously.5,6 Women collected specimens of first morning urine daily from the time they stopped using birth control until the eighth week of clinical pregnancy, or for up to six months if no pregnancy was clinically evident. At the time of urine collection, the women recorded whether or not they had had sexual intercourse or vaginal bleeding in the previous 24 hours. Intercourse that was recorded on a given morning was assumed to have occurred the previous day. Urine collection was complete for 98 percent of the days in the study.

Laboratory Methods

We used the urine specimens to monitor the ovarian steroid changes that accompany ovulation. Assays of estrone 3-glucuronide (a metabolite of estradiol) and pregnanediol 3-glucuronide (a metabolite of progesterone) were carried out by direct radioimmunoassay.7,8 For eight cycles with ambiguous results on radioimmunoassay, data were supplemented by immunofluorometric assays.9,10

The ratio of urinary estrogen metabolites to progesterone metabolites has been discussed by Royston as a way to identify the day of ovulation.11 We refined Royston's algorithm, exploiting the rapid drop in the estrogen-to-progesterone ratio that signals the luteinization of the ovarian follicle.12 Using data on hormones from another study, we found that the day of ovulation selected by the algorithm was highly concordant with the peak urinary concentration of luteinizing hormone (which corresponds approximately with the day of ovulation13). Our data suggest that this method may be approximately equivalent in accuracy to methods based on the surge of serum luteinizing hormone.9

Pregnancy was identified in this study by the measurement of intact human chorionic gonadotropin (hCG) in urine with a highly sensitive and specific immunoradiometric assay.14 This assay was able to detect an increase in urinary hCG as early as six days after ovulation, at the expected time of implantation.

Figure 1. Figure 1. Serial Urinary Hormone Values in a Woman Trying to Conceive. The shaded bars show days of menstrual bleeding. The estimated days of ovulation, based on our algorithm, are indicated by the dotted lines. In the top panel, values are provided for estrone 3-glucuronide (E 1 G, solid line) and pregnanediol 3-glucuronide (PdG, dashed line), adjusted for the creatinine concentration. The second panel from the top shows the ratio of E 1 G to PdG. Days on which sexual intercourse occurred are shown in the third panel by tick marks. The bottom panel shows the urinary concentrations of hCG.

Figure 1 shows the hormonal data from one of the participants. During her 19 weeks of participation, the woman in this example had two apparently normal menstrual cycles and then one in which a clinical pregnancy began. The increase in urinary pregnanediol 3-glucuronide in the latter part of each cycle indicates that ovulation took place during all three cycles. There is evidence in the first cycle of a conceptus that did not survive (indicated by the transient increase in hCG). Conception in the third cycle culminated in the birth of a healthy boy.

Statistical Analysis

If there were only one instance of sexual intercourse during each menstrual cycle, the timing of intercourse that led to conception would be obvious. However, there are usually several days on which intercourse is potentially responsible for any given conception (as was the case in Figure 1), a fact that complicates the estimation of the probability of conception on specific days.

In 1969, Barrett and Marshall proposed a method of estimating the daily probability of conception.15 Their model estimates the probability of conception associated with intercourse on specific days in relation to ovulation and implicitly assumes that the timing of intercourse is the only determinant of whether conception occurs. Schwartz et al. expanded this model by adding “cycle viability” as a component of the probability of conception.16 Cycle viability is based on all conditions other than the timing of intercourse that are necessary for conception. Conception cannot occur without, for example, the release of a healthy egg and adequate preparation of the endometrium for implantation. The expanded model allows for the occurrence of some nonviable cycles, but it assumes that in a viable cycle the effects of intercourse on different days are statistically independent. We have proposed a method17 for applying the model of Schwartz et al. that uses widely available statistical software (GLIM18) and allows adjustment for factors other than the timing of sexual intercourse. In the resulting model, the probability of cycle viability corresponds to the proportion of ovulatory menstrual cycles (those during which ovulation occurs) in which conception is potentially possible.

The Study Sample

A total of 221 women provided records for 708 menstrual cycles. In the subsequent hormone assays, five of these cycles were identified as double cycles (i.e., two apparently ovulatory cycles not separated by recorded menses). This may reflect a failure of the women to record an episode of menstrual bleeding or the occurrence of hormonal cycles without menstrual bleeding.19 Thus, 713 cycles were available for analysis. Pregnancy was detected chemically in 199 of these cycles.6,20 Forty-eight of the pregnancies ended within six weeks of the last menstrual period. The remaining 151 pregnancies lasted at least six weeks and were recognized clinically. In 15, the pregnancy was lost before birth; there was 1 molar pregnancy, 1 ectopic pregnancy, and 13 spontaneous abortions before 24 weeks of gestation. The remaining 136 pregnancies ended in live births.

Our analysis was restricted to menstrual cycles for which a day of ovulation could be identified. Five of the 713 menstrual cycles (<1 percent) were frankly anovulatory, with no increase in the urinary concentration of pregnanediol. A day of ovulation could not be identified for 48 other cycles (34 with missing data and 14 with irregular patterns of hormone production), leaving 660 cycles for which the day of ovulation could be specified (93 percent). For 35 of the 660 cycles (5 percent) either data on the timing of intercourse or data on urinary hCG concentrations were missing for crucial days. This left 625 ovulatory cycles in 217 women for the present analysis (88 percent of all cycles; 98 percent of the women). These 625 cycles included those in which conception occurred for 129 of the 136 live births (95 percent) and all 63 of the pregnancy losses.

Since one third of the conceptuses did not survive to delivery, the total probability of conception can be multiplied by two thirds to estimate the probability of conception leading to a successful pregnancy (assuming that the survival of the conceptus is independent of the timing of intercourse).