title: Nuchal translucency measurements are highly correlated in both mono- and dichorionic twin pairs.
abstract: OBJECTIVE
  To establish the distribution of serological and ultrasound first-trimester Down syndrome markers in twins and identify correlations of significance for risk calculation.
  
  
  METHODS
  Nuchal translucency (NT), PAPP-A and betahCG data were extracted from 181 twin pregnancies (31 mono- and 150 dichorionic) with a normal outcome. All pregnancies were consecutively and prospectively included and examined in the Copenhagen First-Trimester Study. The variance of the sum and the difference of log MoM NT values in twin pairs was used to calculate the correlation.
  
  
  RESULTS
  The serological markers did not correlate and were nearly twice the value seen in singleton pregnancies with a median PAPP-A MoM of 2.14 and a median free betahCG MoM of 2.06. Chorionicity was not found to influence the level of biochemical markers. In all twin pairs (r = 0.343, p < 0.001, F-test), as well as mono- (r = 0.404, p = 0.011, F-test) and dichorionic twins (r = 0.316, p < 0.001, F-test) there was a significant correlation between log MoM NT in each pair.
  
  
  CONCLUSIONS
  As the NT values of fetuses in subsequent pregnancies from the same woman do not correlate, the correlation between NTs in twins reflects that the NT is influenced by placental and maternal factors specific for the particular pregnancy, for example, nutrient supply or vascularisation. The correlation may be useful to improve the precision of the prenatal risk assessment for Down syndrome in first-trimester twin pregnancies. The serological markers were elevated in the examined twins as previously described.
authors.affiliation: Department of Obstetrics and Gynecology, Hvidovre Hospital, Copenhagen, Denmark.
authors.forename: Karen R
authors.initials: KR
authors.lastname: Wøjdemann
authors.forename: Severin Olesen
authors.initials: SO
authors.lastname: Larsen
authors.forename: Anne-Cathrine
authors.initials: AC
authors.lastname: Shalmi
authors.forename: Karin
authors.initials: K
authors.lastname: Sundberg
authors.forename: Ann
authors.initials: A
authors.lastname: Tabor
authors.forename: Michael
authors.initials: M
authors.lastname: Christiansen
mesh_terms: D000465:Algorithms; D015415:Biomarkers / Q000097:blood; D018997:Chorionic Gonadotropin, beta Subunit, Human / Q000097:blood; D018568:Crown-Rump Length; D004200:Diseases in Twins / Q000175:diagnosis / Q000000981:diagnostic imaging*; D004314:Down Syndrome / Q000175:diagnosis / Q000000981:diagnostic imaging*; D005260:Female; D005315:Fetal Diseases / Q000175:diagnosis; D006801:Humans; D016014:Linear Models; D048208:Nuchal Translucency Measurement; D011247:Pregnancy; D011266:Pregnancy-Associated Plasma Protein-A / Q000032:analysis; D011296:Prenatal Diagnosis / Q000379:methods*; D018570:Risk Assessment; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D016216:Ultrasonography, Prenatal
chemical_list: D015415:Biomarkers; D018997:Chorionic Gonadotropin, beta Subunit, Human; D011266:Pregnancy-Associated Plasma Protein-A