Natural limits of pregnancy testing in relation to the expected menstrual period

Context Pregnancy test kits routinely recommend testing "as early as the fi rst day of the missed period." However, a pregnancy cannot be detected befo re the blastocyst implants. Due to natural variability in the timing of ovu lation, implantation does not necessarily occur before the expected onset o f next menses. Objective To estimate the maximum screening sensitivity of pregnancy tests when used on the first day of the expected period, taking into account the natural variability of ovulation and implantation. Design and Setting Community-based prospective cohort study conducted in No rth Carolina between 1982 and 1986. Participants Two hundred twenty-one healthy women 21 to 42 years of age who were planning to conceive. Main Outcome Measures Day of implantation, defined by the serial assay of f irst morning urine samples using an extremely sensitive immunoradiometric a ssay for human chorionic gonadotropin (hCG), relative to the first day of t he missed period, defined as the day on which women expected their next men ses to begin, based on self-reported usual cycle length. Results Data were available for 136 clinical pregnancies conceived during t he study, 14 (10%) of which had not yet implanted by the first day of the m issed period. The highest possible screening sensitivity for an hCG-based p regnancy test therefore is estimated to be 90% (95% confidence interval [CI ], 84%-94%) on the first day of the missed period. By 1 week after the firs t day of the missed period, the highest possible screening sensitivity is e stimated to be 97% (95% Cl, 94%-99%). Conclusions In this study, using an extremely sensitive assay for hCG, 10% of clinical pregnancies were undetectable on the first day of missed menses . fn practice, an even larger percentage of clinical pregnancies may be und etected by current test kits on this day, given their reported assay proper ties and other practical limitations.