title: Childhood and adult cancer in twins: evidence from the Utah genealogy.
abstract: OBJECTIVE
  Evidence suggests that the in utero environment may contribute to subsequent development of cancers in childhood and adulthood. Raised levels of estrogen during pregnancy may be the primary in utero etiologic factor. Mothers of twins have higher estrogen levels during pregnancy than mothers of singletons, therefore, assessment of cancer risk in twins may be informative.
  
  
  METHODS
  We conducted a retrospective cohort study of cancer among twin and singleton newborns selected from the Utah Population Database, matched on birth year and sex. Cancer diagnoses were determined by linkage with the Utah state cancer register. Relative rates of all cancers in childhood and in adulthood in twins compared with singletons, and for specific cancers including testicular, breast and melanoma, were calculated using Poisson regression.
  
  
  RESULTS
  Twin (35,271) and singleton (74,199) births were identified, among whom there were 336 and 691 cancer diagnoses, respectively. The relative risk (RR) of childhood cancer in twins compared with singletons was 0.82 [95% confidence interval (CI) 0.55-1.24] and of adult cancer was 1.06 (0.92-1.22). We found nonsignificant increases in risk among adult twins for cancers of the breast, prostate, testis, lymphatic system, thyroid, and large bowel. The largest departures from unity were for testicular cancer (RR 1.47; 95% CI, 0.73-2.95) and melanoma (RR 0.67; 95% CI, 0.42-1.06).
  
  
  CONCLUSIONS
  These results are consistent with the body of evidence suggesting that twins have a reduced risk of cancer in childhood. Although there is no overall differential in adult cancer risk, these data support the hypothesis that the in utero environment may play an important role in specific cancers.
authors.affiliation: Childhood Cancer Research Group, 57 Woodstock Road, Oxford OX2 6HJ, United Kingdom. rachel.neale@ccrg.ox.ac.uk
authors.forename: Rachel E
authors.initials: RE
authors.lastname: Neale
authors.forename: Geraldine
authors.initials: G
authors.lastname: Mineau
authors.forename: David C
authors.initials: DC
authors.lastname: Whiteman
authors.forename: Pat A
authors.initials: PA
authors.lastname: Brownbill
authors.forename: Michael F G
authors.initials: MF
authors.lastname: Murphy
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D015331:Cohort Studies; D004200:Diseases in Twins / Q000453:epidemiology / Q000209:etiology* / Q000235:genetics; D004967:Estrogens / Q000096:biosynthesis*; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D008498:Medical Record Linkage; D008875:Middle Aged; D009369:Neoplasms / Q000453:epidemiology / Q000209:etiology* / Q000235:genetics; D010375:Pedigree; D016012:Poisson Distribution; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects; D012042:Registries; D012189:Retrospective Studies; D018570:Risk Assessment; D014589:Utah / Q000453:epidemiology; D014599:Uterus / Q000502:physiology
chemical_list: D004967:Estrogens