Table 1.

Baseline characteristics (age standardized) according to oral contraceptive use in the BWHS

Oral contraceptive use
NeverEver<5 y ago≥10-y duration
n13,18640,66213,5526,538
Age (y), mean42.138.330.139.4
Age at menarche (y), mean12.312.412.612.3
BMI at age 18 y (kg/m2), mean22.021.321.220.9
BMI (kg/m2), mean28.827.827.327.2
Education (y), %
    ≤1224.017.520.813.4
    13-1534.535.932.133.4
    ≥1641.346.546.953.1
Family history of breast cancer, %6.86.57.66.5
Parity, %
    Nulliparous42.133.438.541.5
    117.222.525.325.3
    219.023.814.321.1
    ≥321.420.121.812.1
Age at first birth (y), parous only, %
    <2037.633.525.332.3
    20-2437.635.635.131.5
    ≥2523.829.934.635.0
Premenopausal, %74.675.880.877.2
Ever FH use, %14.816.79.716.9
Vigorous activity (h/wk), %
    None34.131.529.331.1
    <548.051.554.252.3
    ≥513.013.212.013.6
Alcohol (drinks/wk), %
    <178.973.373.970.2
    1-614.920.219.522.9
    ≥75.15.96.06.2

Abbreviation: FH, female hormones.