Table 6.

OR of being the MZ twin with the highest mammographic density (defined as any fraction of a % or cm2 higher, 5% higher, or 10% higher) associated with various risk factors

Risk factors/traits the MZ pairs were discordant for* (definition of highest mammographic density)PairsGreater percent density
Greater absolute density
OR (95% CI)OR (95% CI)
BMI >2.5 greater
    Any difference in mammographic density880.59 (0.39-0.92)0.68 (0.45-1.05)
    At least 5% difference570.50 (0.29-0.86)0.61 (0.36-1.05)
    At least 10% difference410.37 (0.18-0.73)0.41 (0.21-0.81)
Menarche >1 y earlier
    Any difference1221.02 (0.71-1.47)1.00 (0.70-1.44)
    At least 5% difference780.90 (0.55-1.45)1.06 (0.67-1.69)
At least 10% difference490.73 (0.37-1.41)0.84 (0.43-1.62)
Nulliparous vs parous
    Any difference662.13 (1.23-3.68)1.99 (1.18-3.34)
    At least 5% difference422.91 (1.25-6.81)3.58 (1.59-8.04)
    At least 10% difference286.48 (1.37-30.7)3.85 (1.22-12.1)
Both parous (parity 1-2 vs ≥3)
    Any difference831.44 (0.92-2.24)1.79 (1.14-2.84)
    At least 5% difference562.30 (1.30-4.08)2.19 (1.23-3.90)
    At least 10% difference352.33 (1.10-4.93)2.33 (1.10-4.93)
Alcohol use (>2 drinks/mo vs ≤2 drinks/mo)
    Any difference600.59 (0.34-1.02)0.50 (0.29-0.87)
    At least 5% difference430.53 (0.28-1.03)0.47 (0.24-0.91)
    At least 10% difference280.49 (0.21-1.13)0.47 (0.20-1.08)
Smoke currently vs do not smoke
    Any difference250.99 (0.40-2.47)0.64 (0.25-1.66)
    At least 5% difference180.94 (0.33-2.67)0.61 (0.21-1.78)
    At least 10% difference130.89 (0.28-2.82)0.62 (0.18-2.08)
Premenopause vs postmenopause
    Any difference411.69 (0.80-3.55)1.40 (0.66-2.99)
    At least 5% difference241.60 (0.59-4.34)1.60 (0.59-4.34)
    At least 10% difference,§151.60 (0.33-7.82)1.60 (0.33-7.82)
Both menopausal (current HT use vs none)
    Any difference231.55 (0.66-3.67)1.52 (0.63-3.66)
    At least 5% difference172.34 (0.73-7.52)2.12 (0.70-6.45)
    At least 10% difference,§81.87 (0.28-12.6)0.57 (0.10-3.27)
Both menopausal (current EPT use vs none)
    Any difference380.76 (0.37-1.54)1.15 (0.59-2.27)
    At least 5% difference270.59 (0.23-1.55)1.08 (0.44-2.67)
    At least 10% difference180.41 (0.10-1.72)1.11 (0.31-3.95)
  • NOTE: Analyses restricted to members of identical twin pairs.

  • * Twin pairs that were concordant (or not different by the defined quantity of each factor) were excluded from each analysis. All models are adjusted for age at mammogram and BMI except that analyses for BMI were only adjusted for age.

  • Any difference in mammographic density means that the OR is the OR for having the highest mammographic density within the twin pair.

  • Difference in mammographic density of 5% (or 10%) means that the OR is for having at least 5% (or 10%) higher mammographic density than the co-twin.

  • § Age at mammogram was excluded in the models due to unstable estimates.

  • None includes E only or P only current users.