Table 3.

Association between breast cancer risk factor, zygosity, and frequency of contact between twins

VariableZygosity
Closeness
MZDZWeekly contactLess often
BMI absolute difference
    ≤2.5168 (65.4%)132 (44.6%)202 (56.9%)98 (49.5%)
    ≥2.689 (34.6%)164 (55.4%)153 (43.1%)100 (50.5%)
    P*<0.00010.094
Alcohol use
    Both >2 drinks monthly or ≤2 drinks monthly143 (70.1%)131 (58.0%)189 (68.7%)85 (54.8%)
    Discordant61 (29.9%)95 (42.0%)86 (31.3%)70 (45.2%)
    P*0.0090.004
Smoking
    Both smoke or both do not smoke232 (90.3%)231 (78.0%)305 (85.9%)158 (79.8%)
    Discordant25 (9.73%)65 (22.0%)50 (14.1%)40 (20.2%)
    P*0.00010.062
Menarche
    Within 1 y133 (51.8%)81 (27.4%)144 (40.6%)70 (35.4%)
    ≥1 y difference124 (48.2%)215 (72.6%)211 (59.4%)128 (64.6%)
    P*<0.00010.23
Nulliparity
    Both nulliparous or both parous190 (73.9%)230 (77.7%)261 (73.5%)159 (80.3%)
    Discordant for parity67 (26.1%)66 (22.3%)94 (26.5%)39 (19.7%)
    P*0.300.074
Parity (among parous pairs)
    Both 1-2 or both ≥393 (52.8%)125 (55.6%)141 (57.6%)77 (49.4%)
    Discordant83 (47.2%)100 (44.4%)104 (42.4%)79 (50.6%)
    P*0.590.11
Menopause
    Both pre or post178 (69.3%)197 (66.6%)239 (67.3%)136 (68.7%)
    Discordant42 (16.3%)66 (22.3%)67 (18.9%)41 (20.7%)
    Any unknown37 (14.4%)33 (11.1%)49 (13.8%)21 (10.6%)
    P*0.110.75
Ever use of HT (among postmenopausal pairs)§
    Both never or both ever users96 (86.5%)83 (72.2%)114 (78.6%)65 (80.2%)
    Discordant15 (13.5%)32 (27.8%)31 (21.4%)16 (19.8%)
    P*0.0080.77
  • NOTE: Numbers do not add up due to missing information on alcohol use (53 MZ pairs and 70 DZ pairs) and hormone use (2 DZ pairs).

  • * χ2 P value.

  • Among 176 MZ and 225 DZ parous pairs.

  • Unknown categories are excluded in the χ2 tests.

  • § Among 111 MZ and 117 DZ postmenopausal pairs.