title: Duration and variability of normal pregnancy. Implications for clinical practice.
abstract: OBJECTIVE
  To estimate the true biologic length and variability of normal pregnancy on the basis of early ultrasonography and to assess the implications for clinical practice.
  
  
  METHODS
  We reviewed the clinical case notes on 476 women whose pregnancies were routinely dated by measurement of the biparietal diameter in the second trimester. After excluding abnormal cases, 355 pregnancies were available for analysis. The duration of pregnancy was studied in relation to maternal characteristics and also to induction of labor for postmaturity.
  
  
  RESULTS
  The mean +/- SD for the normal duration of pregnancy were 279.7 and 7.4 days. The length of pregnancy was weakly related to maternal height. Of the 41 women whose labor was induced for postmaturity, only 7 were truly postmature when gestational age was determined by sonography.
  
  
  CONCLUSIONS
  The current definitions of preterm and postterm may need to be revised to allow the increased precision achieved by ultrasound. Inclusion of menstrual data for the determination of gestational age may lead to incorrect clinical decisions.
authors.affiliation: Department of Obstetrics and Gynaecology, Rush Green Hospital, Essex, England.
authors.forename: M
authors.initials: M
authors.lastname: Mongelli
authors.forename: B
authors.initials: B
authors.lastname: Opatola
mesh_terms: D000328:Adult; D005260:Female; D005333:Fetus / Q000502:physiology; D005865:Gestational Age; D006801:Humans; D007751:Labor, Induced; D011247:Pregnancy / Q000502:physiology*; D013997:Time Factors; D016216:Ultrasonography, Prenatal