title: Screening for trisomies in dichorionic twins by measurement of fetal nuchal translucency thickness according to the mixture model.
abstract: OBJECTIVE
  To examine the distribution of fetal nuchal translucency (NT) thickness in dichorionic twins and investigate the effect of the correlation between NT measurements in each twin pair on the performance of screening for trisomies.
  
  
  METHODS
  The distribution of fetal NT for crown-rump length (CRL) was examined in 5646 dichorionic twin pregnancies, including 103 with fetal trisomies 21, 18 or 13. The correlation in fetal NT in each euploid twin pregnancy was estimated.
  
  
  RESULTS
  The distribution of NT in both euploid and trisomic fetuses was consistent with the mixture model in singleton pregnancies. In the euploid pregnancies, there was a correlation in log NT measurements in each twin pair (r = 0.42, 95% CI: 0.39-0.45) and, after removal of the effect of the operator, this correlation was reduced to 0.34. Allowing for this correlation in risk assessment for trisomies had a major impact on the estimated patient-specific risk but had little effect on the overall performance of screening.
  
  
  CONCLUSIONS
  In dichorionic twin pregnancies, the mixture model of distributions of NT can be applied as in singletons. In screening for trisomies, the correlation in NT measurements between the fetuses should be taken into account in the estimation of patient-specific risks.
authors.affiliation: Department of Mathematics and Statistics, University of Plymouth, Plymouth, UK.
authors.forename: David
authors.initials: D
authors.lastname: Wright
authors.forename: Argyro
authors.initials: A
authors.lastname: Syngelaki
authors.forename: Ismini
authors.initials: I
authors.lastname: Staboulidou
authors.forename: Jader de Jesus
authors.initials: Jde J
authors.lastname: Cruz
authors.forename: Kypros H
authors.initials: KH
authors.lastname: Nicolaides
mesh_terms: D000293:Adolescent; D000328:Adult; D025063:Chromosome Disorders / Q000000981:diagnostic imaging; D002882:Chromosomes, Human, Pair 13 / Q000000981:diagnostic imaging; D002887:Chromosomes, Human, Pair 18; D018568:Crown-Rump Length; D004200:Diseases in Twins / Q000000981:diagnostic imaging*; D004314:Down Syndrome / Q000000981:diagnostic imaging; D005260:Female; D006801:Humans; D008875:Middle Aged; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D014314:Trisomy / Q000175:diagnosis*; D000073839:Trisomy 13 Syndrome; D014427:Twins