title: Second to fourth digit ratio (2D:4D), breast cancer risk factors, and breast cancer risk: a prospective cohort study.
abstract: BACKGROUND
  We aimed to assess whether 2D:4D measures are associated with breast cancer risk.
  
  
  METHODS
  We derived the ratio of the lengths of the index and ring fingers (2D:4D), and right minus left 2D:4D (Δ(r-l)) from digit lengths measured from photocopies of participants' hands collected during a recent follow-up of the Melbourne Collaborative Cohort Study, a prospective study including 24 469 women. Of the 9044 women with available data, we identified 573 incident breast cancer cases. Hazard ratios (HR) and 95% confidence intervals (CI) for a one standard deviation difference in 2D:4D measures were obtained from Weibull survival models, and linear regression models were used to examine potential associations between 2D:4D measures and age at menarche and menopause.
  
  
  RESULTS
  We found a direct association between left 2D:4D and breast cancer risk, an inverse association between Δ(r-l) and risk of breast cancer, but no association between right 2D:4D and breast cancer risk. Among breast cancer cases, both right 2D:4D and Δ(r-l) were inversely associated with age at diagnosis. We also observed associations between both right 2D:4D and Δ(r-l) and age at menopause, with increasing digit ratio measures related to earlier mean age at menopause.
  
  
  CONCLUSIONS
  Digit ratio measures might be associated with breast cancer risk and age at onset of breast cancer. If confirmed in other studies, this suggests that lower exposure or sensitivity to prenatal testosterone might be associated with lower risk of breast cancer.
authors.affiliation: Cancer Epidemiology Centre, Cancer Council Victoria, 100 Drummond Street, Carlton South, Victoria 3053, Australia. david.muller@cancervic.org.au
authors.forename: D C
authors.initials: DC
authors.lastname: Muller
authors.forename: L
authors.initials: L
authors.lastname: Baglietto
authors.forename: J T
authors.initials: JT
authors.lastname: Manning
authors.forename: C
authors.initials: C
authors.lastname: McLean
authors.forename: J L
authors.initials: JL
authors.lastname: Hopper
authors.forename: D R
authors.initials: DR
authors.lastname: English
authors.forename: G G
authors.initials: GG
authors.lastname: Giles
authors.forename: G
authors.initials: G
authors.lastname: Severi
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001315:Australia / Q000453:epidemiology; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology; D015331:Cohort Studies; D005260:Female; D005385:Fingers / Q000033:anatomy & histology*; D006801:Humans; D008875:Middle Aged; D011446:Prospective Studies; D012307:Risk Factors