title: Evaluation of fetal femur length to detect Down syndrome in a Thai population.
abstract: OBJECTIVE
  To assess the value of femur length shortening for prenatal detection of Down syndrome in a Thai population.
  
  
  METHODS
  A prospective study was performed by experienced perinatologists on 3137 women undergoing second-trimester amniocentesis, between 16 and 24 weeks of gestation, for the indications of advanced maternal age and past history of chromosomal abnormality. Biparietal diameter and femur length measurements were obtained before the procedures. Regression equations relating biparietal diameter to femur length were used to calculate observed femur length/expected femur length ratio in the chromosomally normal and Down syndrome fetuses. Sensitivity, specificity, false-positive rate and likelihood ratio of a positive test result at various observed femur length/expected femur length ratios for detection of Down syndrome were calculated. A receiver-operator characteristic curve was used to determine threshold screening ratio.
  
  
  RESULTS
  There were 3084 chromosomally normal pregnancies, 26 fetuses with Down syndrome (1:118), and 27 other chromosomal abnormalities. The relationship between femur length and biparietal diameter (BPD) was: expected femur length=-7.631+0.814 BPD, R(2)=0.78, P<0.001). Femur length in Down syndrome fetuses was significantly shorter than in normal fetuses (P<0.001). A ratio of 0.91 for observed femur length/expected femur length yielded a sensitivity of 42.3%, specificity of 86.2%, false positive rate of 13.8% and likelihood ratio of a positive test result of 3.07 (95% CI 1.94-4.84) for detection of Down syndrome.
  
  
  CONCLUSIONS
  In this study, femur length shortening in the second trimester appears to be a useful screening parameter for fetal Down syndrome in a Thai population.
authors.affiliation: Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Faculty of Medicine, King Chulalongkorn Memorial Hospital, Chulalongkorn University, Bangkok, Thailand. mfmchula@ksc.th.com
authors.forename: Y
authors.initials: Y
authors.lastname: Tannirandorn
authors.forename: S
authors.initials: S
authors.lastname: Manotaya
authors.forename: B
authors.initials: B
authors.lastname: Uerpairojkit
authors.forename: S
authors.initials: S
authors.lastname: Tanawattanacharoen
authors.forename: T
authors.initials: T
authors.lastname: Wacharaprechanont
authors.forename: D
authors.initials: D
authors.lastname: Charoenvidhya
mesh_terms: D000328:Adult; D044466:Asian People / Q000235:genetics*; D004314:Down Syndrome / Q000175:diagnosis* / Q000000981:diagnostic imaging / Q000235:genetics*; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging* / Q000196:embryology*; D006801:Humans; D008423:Maternal Age; D011237:Predictive Value of Tests; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D018566:Pregnancy, High-Risk; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D013785:Thailand; D016216:Ultrasonography, Prenatal / Q000592:standards*