title: Influence of chorionicity on the heritability estimates of blood pressure: a study in twins.
abstract: BACKGROUND
  A basic assumption of the twin design is that environmental influences including prenatal experiences are equal across twin types. However, the intra-uterine environment may differ according to the chorionicity of the monozygotic twins, which may have biased previous heritability estimates of blood pressure.
  
  
  OBJECTIVE
  The aim of the present study was to assess whether the heritability of blood pressure, derived from measurements in monozygotic and dizygotic twins, differs according to the chorionicity of the monozygotic twins.
  
  
  METHODS
  Conventional and 24-h ambulatory blood pressures were measured in 125 dizygotic twin pairs and in 97 dichorionic and 128 monochorionic monozygotic twin pairs at the age of 18-34 years. The twin sample was drawn from the East Flanders Prospective Twin Survey, in which perinatal data were collected at birth. Intra-pair correlation coefficients were calculated and compared between both types of monozygotic twin pairs. Heritability was estimated from model-fitting and path analysis, based on the dizygotic twins and, respectively, all monozygotic twins and the two subtypes.
  
  
  RESULTS
  Intra-pair correlation coefficients for the various blood pressures, after adjustment for body mass index, ranged from 0.45 to 0.71 in the monozygotic twin pairs and did not differ significantly according to chorionicity. Heritability estimates of blood pressure were between 52 and 64%, and were similar when calculated from dizygotic twins and, respectively, dichorionic and monochorionic monozygotic twins.
  
  
  CONCLUSIONS
  Heritability estimates of conventional and ambulatory blood pressure do not differ significantly according to the chorionicity of the monozygotic twins.
authors.affiliation: Hypertension and Cardiovascular Rehabilitation Unit, Faculty of Medicine, University of Leuven, KU Leuven, Belgium. robert.fagard@uz.kuleuven.ac.be
authors.forename: Robert H
authors.initials: RH
authors.lastname: Fagard
authors.forename: Ruth J
authors.initials: RJ
authors.lastname: Loos
authors.forename: Gaston
authors.initials: G
authors.lastname: Beunen
authors.forename: Catherine
authors.initials: C
authors.lastname: Derom
authors.forename: Robert
authors.initials: R
authors.lastname: Vlietinck
mesh_terms: D000293:Adolescent; D000328:Adult; D001724:Birth Weight; D001794:Blood Pressure / Q000235:genetics*; D018660:Blood Pressure Monitoring, Ambulatory; D002823:Chorion; D005260:Female; D006339:Heart Rate / Q000235:genetics; D006801:Humans; D006973:Hypertension / Q000175:diagnosis / Q000235:genetics*; D008297:Male; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic