Table 2.

Breast cancer risk factors and social closeness in 1,106 individual twins by zygosity

CharacteristicMZ (n = 514; 257 pairs)
DZ (n = 592; 296 pairs)
P*
Mean (SD)Mean (SD)
Age at interview (y)51.6 (4.9)51.3 (5.2)0.30
    Range43-6943-75
Age at mammography (y)50.5 (5.0)50.0 (5.3)0.15
    Range40-6739-74
Interval between interview and mammography (y)1.2 (1.2)1.3 (1.2)0.22
No. mammograms in last 5 y3.4 (1.6)3.4 (1.5)0.96
BMI25.9 (5.8)25.8 (5.6)0.78
Age at menarche (y)12.4 (1.2)12.4 (1.2)0.45
No. live births (among parous women)2.7 (1.3)2.8 (1.4)0.21
Percentage of dense tissue (percent mammographic density)29.7 (20.1)29.8 (21.6)0.94
Difference in percent mammographic density between twins10.7 (9.4)18.7 (15.0)<0.0001
Absolute dense tissue (cm2)31.9 (24.7)32.9 (26.6)0.52
Difference in absolute dense tissue between twins (cm2)13.0 (14.4)20.4 (20.2)<0.0001
Absolute nondensity (cm2)91.7 (57.6)95.2 (64.7)0.34
Difference in absolute nondense tissue between twins (cm2)29.5 (30.2)53.1 (48.1)<0.0001

n with characteristic (%)
n with characteristic (%)

European ethnicity502 (97.7%)574 (97.0%)0.47
Parous419 (81.5%)516 (87.2%)0.010
Alcohol, >2 drinks in the last month238 (52.5%)278 (54.3%)0.58
Smoking, smoked within last 6 mo49 (9.5%)93 (15.7%)0.002
Postmenopausal0.16
    Premenopausal186 (36.2%)242 (40.9%)
    Postmenopausal288 (56.0%)314 (53.0%)
    Unknown§40 (7.8%)36 (6.1%)
Ever hormone use (among postmenopausal women)241 (83.7%)257 (82.6%)0.73
    Type of hormone (among ever hormone users)
        % most recent contained E only124 (53.7%)117 (48.0%)0.40
        % most recent contained P only17 (7.4%)17 (7.0%)
        % most recent contained E+P90 (39.0%)110 (45.1%)
Hormone use at mammogram (among postmenopausal women)197 (71.9%)207 (68.1%)0.32
    Type of hormone (among current hormone users)
        % E only102 (52.6%)101 (50.3%)0.50
        % P only14 (7.2%)10 (5.0%)
        % E+P78 (40.2%)90 (44.8%)
Closeness, no. twin pairs report being close
    Both report being close173 (67.8%)139 (47.9%)
    One report being close28 (11.0%)40 (13.8%)
    Both report not being close54 (21.2%)111 (38.3%)<0.0001
  • NOTE: Numbers do not add up due to missing information on number of mammograms in the last 5 y (1 MZ and 1 DZ individual), alcohol use (61 MZ and 80 DZ individuals), ever hormone use (3 DZ individuals), type of most recent hormone (9 MZ and 13 DZ individuals), hormone use at mammogram (14 MZ and 10 DZ individuals), type of current hormone (3 MZ and 6 DZ individuals), and closeness (2 MZ pairs and 6 DZ pairs).

  • * P value from t test to compare means or χ2 to compare frequencies.

  • Among 419 MZ and 516 DZ parous individuals.

  • Absolute difference in percent and absolute mammographic density and absolute nondense tissue (mammographic density in twin with highest density minus mammographic density in twin with lowest density).

  • § Unknown category is excluded in the χ2 test.