title: Sex differences in birth defects: a study of opposite-sex twins.
abstract: BACKGROUND
  Sex differences in structural birth defects are often confounded by environmental risk factors. Opposite-sex twins provide a unique model for detecting sex differences in birth defects while maximally controlling environmental risk factors in a natural setting.
  
  
  METHODS
  Population data from the Florida Birth Defects Registry were analyzed. A total of 4,768 pairs of twins who were discordant for sex and born between 1996 and 2001 were analyzed. The McNemar test was used to compare the differences between a male twin and his twin sister for the risk of developing specific defects and organ-system defects.
  
  
  RESULTS
  Of 4,768 twin pairs, 225 males (4.72%) and 175 females (3.67%) had birth defects. Among opposite-sex twin pairs, males had a 29% higher risk for birth defects than their twin sisters. Compared to their twin sisters, males had a 5.4 times higher risk for pyloric stenosis and a 2.4 times higher risk for obstructive genitourinary defect, but only one-tenth the risk for congenital hip dislocation.
  
  
  CONCLUSIONS
  Sex differences in birth defects exist between opposite-sex twins.
authors.affiliation: Division of Neonatology, Department of Pediatrics, College of Medicine, University of Florida, Gainesville, 32610-0296, USA. terrycui@yahoo.com
authors.forename: Wei
authors.initials: W
authors.lastname: Cui
authors.forename: Chang-Xing
authors.initials: CX
authors.lastname: Ma
authors.forename: Yiwei
authors.initials: Y
authors.lastname: Tang
authors.forename: Vivian
authors.initials: V
authors.lastname: Chang
authors.forename: P V
authors.initials: PV
authors.lastname: Rao
authors.forename: Mario
authors.initials: M
authors.lastname: Ariet
authors.forename: Michael B
authors.initials: MB
authors.lastname: Resnick
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Roth
mesh_terms: D000013:Congenital Abnormalities / Q000453:epidemiology* / Q000473:pathology; D005260:Female; D006801:Humans; D008297:Male; D012189:Retrospective Studies; D012307:Risk Factors; D012737:Sex Factors; D014427:Twins