title: Risk of oral clefts in twins.
abstract: BACKGROUND
  Small studies have indicated that twinning increases the risk of oral cleft.
  
  
  METHODS
  We used data from a Danish national population-based cohort study to investigate whether twinning was associated with isolated oral cleft, and to estimate the twin probandwise concordance rate and heritability. Twins (207 affected/130,710) and singletons (7766 affected/4,798,526) born from 1936 through 2004 in Denmark were ascertained by linkage among the Danish Facial Cleft Database, the Danish Twin Registry, and the Civil Registration System. We computed oral cleft prevalence and prevalence proportion ratio for twins versus singletons, stratified for 3 subphenotypes. Probandwise concordance rates and heritability for twins were estimated for 2 phenotypes--cleft lip with or without cleft palate (CL/P) and cleft palate (CP).
  
  
  RESULTS
  The prevalence of oral cleft was 15.8 per 10,000 twins and 16.6 per 10,000 singletons (prevalence proportion ratio = 0.95; 95% confidence interval = 0.83-1.1). This prevalence was similar for monozygotic and dizygotic twins. The probandwise concordance rate was higher for CL/P for monozygotic twins than for dizygotic twins (50% vs. 8%, respectively). A similar contrast was present for CP. Recurrence risk for both types of clefts was greater in dizygotic twins than in non-twin siblings. Heritability estimates were above 90% for both CL/P and CP.
  
  
  CONCLUSIONS
  No excess risk of oral cleft could be demonstrated for twins compared with singletons. The concordance rates and heritability estimates for both types of clefts show a strong genetic component.
authors.affiliation: Department of Epidemiology, Institute of Public Health, University of Southern Denmark, Odense, Denmark. dgrosen@health.sdu.dk
authors.forename: Dorthe
authors.initials: D
authors.lastname: Grosen
authors.forename: Camilla
authors.initials: C
authors.lastname: Bille
authors.forename: Inge
authors.initials: I
authors.lastname: Petersen
authors.forename: Axel
authors.initials: A
authors.lastname: Skytthe
authors.forename: Jacob von Bornemann
authors.initials: Jv
authors.lastname: Hjelmborg
authors.forename: Jacob Krabbe
authors.initials: JK
authors.lastname: Pedersen
authors.forename: Jeffrey Clark
authors.initials: JC
authors.lastname: Murray
authors.forename: Kaare
authors.initials: K
authors.lastname: Christensen
mesh_terms: D002971:Cleft Lip / Q000453:epidemiology / Q000235:genetics* / Q000601:surgery; D016208:Databases, Factual; D003718:Denmark / Q000453:epidemiology; D004200:Diseases in Twins / Q000453:epidemiology / Q000235:genetics*; D005500:Follow-Up Studies; D020022:Genetic Predisposition to Disease / Q000453:epidemiology*; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D015995:Prevalence; D012016:Reference Values; D012042:Registries; D018570:Risk Assessment; D017678:Sex Distribution; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic