title: Screening for trisomy 18 by maternal age, fetal nuchal translucency, free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A.
abstract: OBJECTIVE
  To derive a model and examine the performance of first-trimester screening for trisomy 18 by maternal age, fetal nuchal translucency (NT) thickness, and maternal serum free beta-human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A).
  
  
  METHODS
  Prospective combined screening for trisomy 21 was performed at 11 + 0 to 13 + 6 weeks in 56 893 singleton pregnancies, including 56 376 cases of euploid fetuses, 395 with trisomy 21 and 122 with trisomy 18. The measured free beta-hCG and PAPP-A were converted into a multiple of the median (MoM) and then into likelihood ratios (LR). Similarly, the measured NT was transformed into LRs using the mixture model of NT distributions. In each case the LRs for NT and the biochemical markers were multiplied by the age and gestation-related risk to derive the risk for trisomy 21 and trisomy 18. Detection rates (DRs) and false-positive rates (FPRs) were calculated by taking the proportions with risks above a given risk threshold.
  
  
  RESULTS
  In screening with the algorithm for trisomy 21, at a FPR of 3%, the estimated DRs of trisomies 21 and 18 were 89% and 82%, respectively. The use of an algorithm for trisomy 18 identified 93% of affected fetuses at a FPR of 0.2%. When the algorithm for trisomy 21 was used and screen positivity was fixed at a FPR of 3%, and in addition the algorithm for trisomy 18 was used and screen positivity was fixed at a FPR of 0.2%, the overall FPR was 3.1% and the DRs of trisomies 21 and 18 were 90% and 97%, respectively.
  
  
  CONCLUSIONS
  A beneficial side effect of first-trimester combined screening for trisomy 21 is the detection of a high proportion of fetuses with trisomy 18. If an algorithm for trisomy 18 in addition to the one for trisomy 21 is used, more than 95% of trisomy 18 fetuses can be detected with a minor increase of 0.1% in the overall FPR.
authors: Harris Birthright Research Centre for Fetal Medicine, King's College Hospital, London, UK.
authors: K O
authors: KO
authors: Kagan
authors: D
authors: D
authors: Wright
authors: N
authors: N
authors: Maiz
authors: I
authors: I
authors: Pandeva
authors: K H
authors: KH
authors: Nicolaides
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D015415:Biomarkers / Q000097:blood; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood*; D002887:Chromosomes, Human, Pair 18 / Q000235:genetics*; D004314:Down Syndrome / Q000175:diagnosis; D005189:False Positive Reactions; D005260:Female; D005315:Fetal Diseases / Q000175:diagnosis*; D006801:Humans; D008403:Mass Screening / Q000379:methods; D008423:Maternal Age; D008875:Middle Aged; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011266:Pregnancy-Associated Plasma Protein-A / Q000032:analysis*; D011296:Prenatal Diagnosis / Q000379:methods*; D011446:Prospective Studies; D014314:Trisomy / Q000175:diagnosis*; D055815:Young Adult
chemical_list: D015415:Biomarkers; D018997:Chorionic Gonadotropin, beta Subunit, Human; D011266:Pregnancy-Associated Plasma Protein-A