title: Femur length and trisomy 21: impact of gestational age on screening efficiency.
abstract: OBJECTIVE
  This study assesses two methods used to define relatively short femur in screening for trisomy 21 and examines changes in performance of screening with gestational age.
  
  
  METHODS
  Retrospective analysis of data on menstrual age, femur length (FL) and biparietal diameter (BPD) in 49 trisomy 21 pregnancies and 6069 normal controls. Reference ranges were derived for BPD/FL versus menstrual age and for FL versus BPD. Two methods of defining short femur (BPD/FL and observed-to-expected FL ratio) were examined for false-positive rates and detection rates for trisomy 21 at different gestational ages.
  
  
  RESULTS
  In the control group the BPD/FL ratio and its standard deviation decreased with menstrual age. Trisomy 21 was associated with a significantly higher BPD/FL ratio (P < 0.001) and the deviation increased significantly with menstrual age (P < 0.05). Eleven percent of 28 fetuses examined at 15-17 weeks had a BPD/FL above the 95th centile compared with 24% of 21 fetuses examined at 18-20 weeks (P = 0.40). The median observed-to-expected FL ratio in the control group was 1.0 throughout the gestational age range but the standard deviation decreased significantly with menstrual age (P < 0.01). Trisomy 21 was associated with a significantly reduced observed-to-expected FL ratio (P < 0.001) and the deviation increased significantly with menstrual age (P < 0.05). A fixed cut-off of 0.91 for observed-to-expected FL ratio provided a false-positive rate of 12% at 15-17 weeks compared with 6% at 18-20 weeks of gestation (P < 0.001) with detection rates of 29 and 38%, respectively (P = 0.73).
  
  
  CONCLUSIONS
  Irrespective of the definition used to define the condition, relatively short femur is a poor marker for trisomy 21 particularly when the assessment takes place before 18 weeks of gestation.
authors: Department of Obstetrics and Gynecology, Cedars-Sinai Medical Center, Cedars-Sinai Medical Center Burns & Allen Research Institute, UCLA School of Medicine, Los Angeles, CA, USA.
authors: R J
authors: RJ
authors: Snijders
authors: L D
authors: LD
authors: Platt
authors: N
authors: N
authors: Greene
authors: D
authors: D
authors: Carlson
authors: D
authors: D
authors: Krakow
authors: K
authors: K
authors: Gregory
authors: K
authors: K
authors: Bradley
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D004314:Down Syndrome / Q000175:diagnosis / Q000000981:diagnostic imaging* / Q000453:epidemiology; D005314:Embryonic and Fetal Development; D005260:Female; D005269:Femur / Q000196:embryology / Q000254:growth & development*; D005865:Gestational Age; D006801:Humans; D015994:Incidence; D008403:Mass Screening / Q000379:methods; D008875:Middle Aged; D011237:Predictive Value of Tests; D011247:Pregnancy; D012016:Reference Values; D012044:Regression Analysis; D012189:Retrospective Studies; D012307:Risk Factors; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000379:methods*