title: The observed vs. expected lung-to-head ratio does not correct for the effect of gestational age on the lung-to-head ratio.
abstract: OBJECTIVE
  To assess whether the observed vs. expected lung-to-head ratio (o/e LHR) corrects for the dependence of the LHR on gestational age.
  
  
  METHODS
  Published data on right lung area (LA) and LHR were used to plot the 50th percentile and different fixed values of the o/e LHR (e.g. 30%) against gestational age from 16-32 weeks. The Z-scores for various fixed o/e LHR values and similar percent value of LA were calculated. The effect of using a fixed LHR or a fixed o/e LHR was tested against gestational age. The o/e LHR-equivalent to a fixed LHR of 1.0 was assessed against gestational age.
  
  
  RESULTS
  The LHR and the o/e LHR both increase with gestational age. The Z-score of a given fixed value of the o/e LHR (e.g. 30%) is not similar to the Z-score of the same percent (e.g., 30%) of the expected LA, and thus identifies different proportions of subjects. A fixed o/e LHR (e.g. 30%) results in different populations, depending on the gestational age. The o/e LHR equivalent to an LHR value of 1.0 decreases from 80% at 16 weeks to 30% at 32 weeks.
  
  
  CONCLUSIONS
  The o/e LHR is not independent of gestational age. Studies using this parameter should be interpreted with caution.
authors.affiliation: Department of Obstetrics and Gynecology, Miller School of Medicine, University of Miami, Miami, FL 33136, USA. rquintero@med.miami.edu
authors.forename: Rubén A
authors.initials: RA
authors.lastname: Quintero
authors.forename: Eftichia V
authors.initials: EV
authors.lastname: Kontopoulos
authors.forename: Luis F
authors.initials: LF
authors.lastname: Quintero
authors.forename: David C
authors.initials: DC
authors.lastname: Landy
authors.forename: Reinaldo
authors.initials: R
authors.lastname: Gonzalez
authors.forename: Ramen H
authors.initials: RH
authors.lastname: Chmait
mesh_terms: D000886:Anthropometry; D005260:Female; D005865:Gestational Age; D006548:Hernia, Diaphragmatic / Q000000981:diagnostic imaging; D065630:Hernias, Diaphragmatic, Congenital; D006801:Humans; D011247:Pregnancy; D016216:Ultrasonography, Prenatal / Q000592:standards