Table 1.

Characteristics of monozygous and dizygous female twin subjects

Monozygous (n = 571), mean (SE)Dizygous (n = 380), mean (SE)
Age at interview (y)50.72 (0.33)53.03 (0.38)
Age at mammogram (y)50.65 (0.32)52.55 (0.38)
Time between interview and mammogram (mo)6.29 (0.23)6.14 (0.28)
Weight (kg)66.18 (0.50)67.87 (0.61)
Height (cm)162.00 (0.26)162.97 (0.32)
Age at menarche (y)13.04 (0.06)12.93 (0.07)
Age at first birth (y; parous women only)24.88 (0.14)25.07 (0.17)
No. live births2.31 (0.05)2.42 (0.06)
Years of oral contraceptive use6.04 (0.22)5.87 (0.27)
Years of hormone replacement therapy2.46 (0.17)2.05 (0.21)
Pack-years of smoking5.58 (0.43)6.97 (0.52)
PMD37.82 (0.79)35.69 (0.96)
Absolute dense area of mammographic tissue (cm2)40.50 (0.96)38.33 (1.17)
Absolute nondense area of mammographic tissue (cm2)78.60 (2.03)84.78 (2.47)
Total area of the breast (cm2)119.14 (1.98)123.05 (2.40)
Parous (% yes)84.685.9
Cessation of periods (% stopped)59.355.0