title: The case against using ordinal numbers for gestational age.
abstract: OBJECTIVE
  Use of ordinal numbers (eg, twelfth) instead of cardinal numbers (eg, twelve) to measure gestational age often leads to clinical confusion. We conducted this study to document the prevalence of ambiguous or contradictory use of ordinal numbers for gestational age and to discuss some clinical implications.
  
  
  METHODS
  We reviewed a convenience sample of standard texts in obstetrics and in abortion and examined a random sample of articles on abortion.
  
  
  RESULTS
  Imprecise or incorrect use of ordinal numbers for gestational age was common: Eight of nine (89%) obstetrics texts and all six abortion texts had this problem. The corresponding figure for the abortion articles was 32 of 88 (36%).
  
  
  CONCLUSIONS
  Use of ordinal numbers for gestational age introduces information bias (misclassification) into the scientific literature. More importantly, it may lead to clinical errors related to the timing of administration of antenatal corticosteroids and the upper limit for induced abortions. Gestational age measurements should use only cardinal numbers (eg, twelve) of completed days or weeks from the last menstrual period. Clinicians should abandon use of ordinal numbers for gestational age.
authors.affiliation: Department of Obstetrics, Gynecology and Reproductive Sciences, University of California, San Francisco, USA.
authors.forename: S
authors.initials: S
authors.lastname: Amersi
authors.forename: D A
authors.initials: DA
authors.lastname: Grimes
mesh_terms: D015982:Bias; D005865:Gestational Age; D006801:Humans; D008433:Mathematics