Table 2.

Adjusted and unadjusted mean % breast density at screening nearest to second health check by potential confounding factors (n = 1,413)

n (%)Mean % breast density (95% CI)Univariate P*Adjusted P*,
BMI (kg/m2)
    16.7-22.9236 (16.7)38.1 (35.0-41.2)<0.001<0.001
    >22.9-24.6235 (16.6)30.3 (27.5-33.0)
    >24.6-26.1236 (16.7)25.3 (22.7-27.9)
    >26.1-28.0235 (16.6)26.0 (23.4-28.7)
    >28.0-30.8236 (16.7)20.7 (18.1-23.3)
    >30.8-48.4235 (16.6)14.4 (12.2-16.5)
Age at second health check (y)
    55-59341 (24.1)29.9 (27.5-32.3)<0.001
    60-64657 (46.5)24.8 (23.2-26.5)
    65-69289 (20.5)23.8 (21.4-26.3)
    70+126 (8.9)24.3 (20.4-28.3)
Parity
    0217 (15.4)33.4 (30.2-36.5)<0.001<0.001
    1167 (11.8)29.6 (26.0-33.2)
    2577 (40.8)25.5 (23.8-27.2)
    3304 (21.5)23.5 (21.0-25.9)
    4+148 (10.5)16.5 (13.8-19.2)
Age at menarche (y)
    9-11283 (20.9)22.0 (19.5-24.4)<0.0010.002
    12-13554 (41.1)25.3 (23.5-27.1)
    14+514 (39.1)28.8 (26.8-30.7)
Years since menopause
    <578 (5.5)30.7 (25.8-35.6)0.0010.005
    5-9295 (20.9)28.8 (26.3-31.3)
    10-14354 (25.1)23.7 (21.5-26.0)
    15-19382 (27.0)25.1 (22.8-27.4)
    20+304 (21.5)24.9 (22.5-27.4)
Age at birth of first child (y)
    16-1966 (4.7)18.6 (14.1-23.2)<0.0010.371
    20-24545 (38.6)23.7 (21.9-25.5)
    25-29425 (30.1)25.5 (23.4-27.5)
    30-34121 (8.6)24.9 (21.1-28.6)
    35+256 (18.1)33.1 (30.2-36.0)
Cigarette smoking
    Current115 (8.1)25.5 (21.7-29.3)0.7140.102
    Former419 (29.7)26.4 (24.3-28.5)
    Never879 (62.2)25.6 (24.1-27.0)
Alcohol intake (units/wk)
    0240 (17)24.6 (21.8-27.3)0.4140.644
    >0-2385 (27.2)26.2 (23.9-28.4)
    >2-5327 (23.1)25.2 (22.8-27.6)
    >5-10271 (19.2)25.4 (22.8-27.9)
    >10190 (13.4)28.3 (25.2-31.3)
Years since stopping HRT
    ≤372 (5.1)27.3 (22.5-32.1)0.2230.195
    >3-683 (5.9)31.3 (26.6-35.9)
    >691 (6.5)21.9 (17.5-26.3)
    Never taken HRT1,161 (82.5)25.6 (24.3-26.9)
No. first-degree relatives with breast cancer
    01,288 (91.2)25.7 (24.4-26.9)0.2300.240
    1116 (8.2)26.6 (22.7-30.6)
    29 (0.6)35.5 (19.8-51.2)
  • * Ps from likelihood ratio tests, modeling square root of % dense.

  • Adjusted Ps from model, including sextile of BMI, parity (0, 1, 2, 3, 4+), cigarette smoking status, years since menopause (<5, 5 to <10, 10-14, 15-19, 20+), and age at menarche (9-11, 12-13, 14+).