Mini-review: Prenatal ages and stages - Measures and errors

Background: The confusing term "gestational age" is generally either not de fined or is used for menstrual "age," postovulatory age, or postfertilizati onal 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 morphologi cal meaning in embryology and should not be used either for ages or for len gths. Methods: Embryonic age is best assessed, when possible, from the 23 interna tionally recognized morpho logical stages. A morphological staging system i s 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 t he great est length (GL), exclusive of the lower limbs. These points are di scussed in detail, and justification for the views expressed is provided. Conclusions: The continuing confusion concerning prenatal age is shown to b e unnecessary once the ambiguous and superfluous term "gestational age" is abandoned. (C) 2000 Wiley-Liss, Inc.