Context Pregnancy test kits routinely recommend testing "as early as the first
day of the missed period." However, a pregnancy cannot be detected before
the blastocyst implants. Due to natural variability in the timing of ovulation,
implantation does not necessarily occur before the expected onset of next
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 North 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 first morning urine
samples using an extremely sensitive immunoradiometric assay for human chorionic
gonadotropin (hCG), relative to the first day of the missed period, defined
as the day on which women expected their next menses to begin, based on self-reported
usual cycle length.
Results Data were available for 136 clinical pregnancies conceived during the
study, 14 (10%) of which had not yet implanted by the first day of the missed
period. The highest possible screening sensitivity for an hCG-based pregnancy
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 first day of the
missed period, the highest possible screening sensitivity is estimated to
be 97% (95% CI, 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. In practice,
an even larger percentage of clinical pregnancies may be undetected by current
test kits on this day, given their reported assay properties and other practical