title: Natural limits of pregnancy testing in relation to the expected menstrual period.
abstract: BACKGROUND
  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 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.
  
  
  METHODS
  Community-based prospective cohort study conducted in North Carolina between 1982 and 1986.
  
  
  METHODS
  Two hundred twenty-one healthy women 21 to 42 years of age who were planning to conceive.
  
  
  METHODS
  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 limitations.
authors.affiliation: Epidemiology Branch, MD A3-05, National Institute of Environmental Health Sciences, Durham, NC 27709, USA. wilcox@niehs.nih.gov
authors.forename: A J
authors.initials: AJ
authors.lastname: Wilcox
authors.forename: D D
authors.initials: DD
authors.lastname: Baird
authors.forename: D
authors.initials: D
authors.lastname: Dunson
authors.forename: R
authors.initials: R
authors.lastname: McChesney
authors.forename: C R
authors.initials: CR
authors.lastname: Weinberg
mesh_terms: D000328:Adult; D006063:Chorionic Gonadotropin / Q000652:urine*; D010064:Embryo Implantation; D005260:Female; D006801:Humans; D008597:Menstrual Cycle; D008598:Menstruation; D010060:Ovulation; D011247:Pregnancy; D011258:Pregnancy Tests; D011446:Prospective Studies; D011933:Reagent Kits, Diagnostic; D012648:Self Care; D012680:Sensitivity and Specificity
chemical_list: D006063:Chorionic Gonadotropin; D011933:Reagent Kits, Diagnostic