title: Birth order and oral clefts: a meta analysis.
abstract: BACKGROUND
  There is evidence that late birth order is associated with some complex disorders. For orofacial clefts there is no consensus as to whether increased birth order is associated or not. A meta-analysis of published data on cleft lip or cleft palate (CL/P and CP) was carried out to ascertain whether there is an increased risk for children of high birth order to have an oral cleft.
  
  
  METHODS
  All data available with information regarding the frequency of live births and CL/P and CP cases by birth order (1, 2, 3, and 4 or more) were included in the analysis, and the birth order category "1" was considered to be with no risk (OR = 1.0).
  
  
  RESULTS
  Children with higher birth order are more likely to have CL/P and CP with odds ratios increasing with birth order to a peak of 3.0 in children birth order "4 or more." Results are not different when isolated and syndromic cases are combined.
  
  
  CONCLUSIONS
  CL/P and CP occurrence is correlated with increasing birth order. Further studies, taking into consideration sample size and factors such as income status, race, paternal age, vitamin intake, and social habits, should be done to determine conclusively the association between birth order and oral clefts.
authors.affiliation: Department of Pediatrics, The University of Iowa, Iowa City, Iowa 52242, USA. alexandre-vieira@uiowa.edu
authors.forename: Alexandre R
authors.initials: AR
authors.lastname: Vieira
authors.forename: Iêda M
authors.initials: IM
authors.lastname: Orioli
mesh_terms: D001722:Birth Order; D002648:Child; D002971:Cleft Lip / Q000453:epidemiology / Q000209:etiology*; D002972:Cleft Palate / Q000453:epidemiology / Q000209:etiology*; D015986:Confounding Factors, Epidemiologic; D005260:Female; D006801:Humans; D016017:Odds Ratio; D011247:Pregnancy; D018873:Pregnancy Rate; D012307:Risk Factors