title: Prenatal ages and stages-measures and errors.
abstract: BACKGROUND
  The confusing term "gestational age" is generally either not defined or is used for menstrual "age," postovulatory age, or postfertilizational age. The designation (post)menstrual weeks and/or days is very useful in obstetrics but, because prenatal age extends from fertilization to birth, menstrual "age" is a misnomer. The term "stage" has a specific morphological meaning in embryology and should not be used either for ages or for lengths.
  
  
  METHODS
  Embryonic age is best assessed, when possible, from the 23 internationally recognized morphological stages. A morphological staging system is not available for the fetal period, and fetal age is assessed mainly from measurements.
  
  
  RESULTS
  Among these, the unsatisfactory designation crown-rump length (CRL) should be replaced, in ultrasonic biometry as well as in embryology, by the greatest length (GL), exclusive of the lower limbs. These points are discussed in detail, and justification for the views expressed is provided.
  
  
  CONCLUSIONS
  The continuing confusion concerning prenatal age is shown to be unnecessary once the ambiguous and superfluous term "gestational age" is abandoned.
authors.affiliation: Institut d'Embryologie Spéciale, Université de Fribourg, Fribourg, Switzerland.
authors.forename: R
authors.initials: R
authors.lastname: O'Rahilly
authors.forename: F
authors.initials: F
authors.lastname: Müller
mesh_terms: D018568:Crown-Rump Length; D004622:Embryo, Mammalian / Q000502:physiology*; D005260:Female; D005333:Fetus / Q000502:physiology*; D005865:Gestational Age; D006801:Humans; D008597:Menstrual Cycle; D011247:Pregnancy