Table 4.

Pairwise correlations of mammographic density by zygosity and breast cancer risk factors (n = 1,106; 553 pairs)

NNPercent density
Absolute density
MZDZMZ Sp R (95% CI)DZ Sp R (95% CI)MZ Sp R (95% CI)DZ Sp R (95% CI)
All twins2572960.74 (0.68-0.79)0.38 (0.28-0.47)0.69 (0.61-0.75)0.47 (0.37-0.55)
BMI absolute difference
    <1.190550.73 (0.62-0.81)0.54 (0.31-0.70)0.78 (0.68-0.85)0.59 (0.38-0.74)
    1.1-2.578770.81 (0.72-0.87)0.45 (0.25-0.61)0.76 (0.64-0.84)0.51 (0.32-0.66)
    2.6-5.051770.72 (0.55-0.83)0.29 (0.07-0.48)0.69 (0.50-0.81)0.41 (0.21-0.58)
    >5.038870.43 (0.12-0.66)0.16 (−0.06-0.36)0.36 (0.04-0.61)0.38 (0.18-0.55)
Age at menarche
    Within 1 y133810.72 (0.63-0.79)0.28 (0.07-0.47)0.71 (0.61-0.78)0.44 (0.25-0.60)
    ≥1 y difference1242150.76 (0.67-0.82)0.41 (0.29-0.51)0.67 (0.55-0.75)0.48 (0.37-0.57)
Nulliparity
    Both nulliparous1450.82 (0.50-0.94)10.78 (0.39-0.92)1
    Both parous1762250.73 (0.65-0.79)0.42 (0.31-0.52)0.67 (0.58-0.75)0.50 (0.39-0.59)
    Discordant67660.73 (0.59-0.82)0.12 (−0.13 to 0.35)0.65 (0.48-0.77)0.29 (0.05-0.49)
Parity (among parous pairs)
    Both 1-2 pregnancies47620.73 (0.55-0.84)0.51 (0.30-0.67)0.67 (0.47-0.80)0.53 (0.32-0.69)
    Both ≥3 pregnancies46630.78 (0.63-0.87)0.36 (0.12-0.56)0.67 (0.46-0.80)0.42 (0.19-0.60)
    Discordant831000.70 (0.57-0.80)0.40 (0.22-0.55)0.66 (0.52-0.77)0.53 (0.37-0.66)
Alcohol use*
    Both >2 drinks monthly79800.80 (0.71-0.87)0.34 (0.13-0.52)0.82 (0.73-0.88)0.44 (0.24-0.60)
    Both ≤2 drinks monthly64510.72 (0.58-0.82)0.40 (0.14-0.61)0.66 (0.48-0.77)0.46 (0.20-0.65)
    Discordant61950.71 (0.56-0.82)0.36 (0.17-0.52)0.56 (0.35-0.71)0.41 (0.23-0.56)
Smoking
    Both current smoker12140.79 (0.37-0.93)0.60 (0.09-0.85)0.78 (0.33-0.93)0.42 (−0.15 to 0.77)
    Both do not smoke2202170.75 (0.68-0.80)0.37 (0.25-0.48)0.69 (0.62-0.76)0.49 (0.38-0.58)
    Discordant25650.58 (0.23-0.79)0.30 (0.06-0.51)0.50 (0.12-0.74)0.37 (0.14-0.56)
Menopause
    Both pre67800.66 (0.49-0.77)0.35 (0.14-0.53)0.56 (0.37-0.71)0.47 (0.27-0.62)
    Both post1111170.71 (0.61-0.79)0.38 (0.22-0.53)0.68 (0.56-0.77)0.40 (0.23-0.54)
    Discordant42660.72 (0.52-0.84)0.33 (0.10-0.53)0.76 (0.58-0.86)0.51 (0.30-0.66)
    Any unknown37330.70 (0.48-0.83)0.33 (−0.02 to 0.60)0.70 (0.48-0.83)0.34 (−0.01 to 0.61)
HT use (among postmenopausal pairs)
    Both current HT users64520.64 (0.47-0.77)0.40 (0.14-0.60)0.60 (0.42-0.74)0.39 (0.12-0.60)
    Both nonusers17130.59 (0.13-0.83)0.74 (0.28-0.91)0.65 (0.23-0.86)0.74 (0.28-0.91)
    Discordant for current HT use23460.85 (0.66-0.93)0.25 (−0.05 to 0.50)0.87 (0.70-0.94)0.33 (0.04-0.56)
    Any unknown760.86 (0.23-0.98)0.54 (−0.52 to 0.93)0.64 (−0.26 to 0.93)0.94 (0.49-0.99)
  • Abbreviation: Sp R, Spearman's correlation coefficient.

  • * Fifty-three MZ pairs and 70 DZ pairs were excluded due to missing information on their alcohol use or their co-twin's alcohol use.