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Table 1 Baseline characteristics for 70,575 women in the PLCO Cancer Screening Trial Cohort.

From: Breast cancer epidemiology according to recognized breast cancer risk factors in the Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial Cohort

  Number %
Age at baseline   
   55–59 24,381 34.6
   60–64 21,405 30.3
   65–69 15,405 21.8
   70–74 9,384 13.3
Race/ethnicity   
   White 62,425 88.5
   African-American 4,063 5.8
   Asian/Pacific Islander 2,754 3.9
   Other* 1,333 1.9
Education   
   Less than H.S. 4,598 6.5
   Completed H.S. 28,588 40.5
   Beyond H.S. 37,215 52.7
   Unknown 174 0.3
Marital status   
   Married or living as married 48,749 69.1
   Widowed 9,700 13.7
   Divorced or separated 9,616 12.7
   Never married/missing 2,510 4.5
Mother or sister with breast cancer   
   No 57,214 81.1
   Yes 9,692 13.7
   Unknown 3,669 5.2
Age at menarche   
   < 12 14,259 20.2
   12 – 13 37,816 53.6
   14 – 15 15,105 21.4
   ≥ 16 3,208 4.6
   Missing 187 0.3
Years of oral contraceptive use   
   None 32,170 45.6
   < 1 year 10,219 14.5
   2 – 3 years 7,636 10.8
   4 – 5 years 5,208 7.4
   6 – 9 years 6,303 8.9
   ≥ 10 years 8,830 12.5
   Missing 209 0.3
Parity and age at first birth   
   Nulliparous 6,454 9.1
   1–2   
< 20 years 2,499 3.5
20–24 years 9,336 13.2
25–29 years 6,629 9.4
30–34 years 2,405 3.4
≥ 35 years 1,021 1.5
   3–4   
< 20 years 5,721 8.1
20–24 years 16,227 23.0
25–29 years 5,942 8.4
30–34 years 1,020 1.5
≥ 35 years 343 0.5
   ≥ 5   
< 20 years 3,818 5.4
20–24 years 7,162 10.2
25–29 years 1,506 2.1
30–34 years 193 0.3
≥ 35 years 194 0.3
   Missing or unknown 105 0.2
Menopause Type   
   Natural 43,572 61.7
   Surgical 23,312 3233.0
   Radiation or medications 2,346 3.3
   Missing 1345 41.9
Age at Natural Menopause   
   < 45 years 4,468 6.3
   45–49 years 11,175 15.8
   50–54 years 22,134 31.4
   55+ years 5,709 8.1
   Missing 86 0.1
Menopausal hormone therapy use   
   None 23,262 33.0
   Former use 11,435 16.2
   Current use, ≤ 5 years 11,546 16.4
   Current use, > 5 years 23,712 33.6
   Missing 620 0.9
Body Mass Index (BMI; kg/m2)   
   < 18.5 892 1.3
   18.5 – 24.9 27,251 38.6
   25 – 29 24,214 34.3
   30 – 34.9 11,102 15.7
   ≥ 35 6,297 8.9
   Missing 819 1.2
  1. *Other race/ethnicity includes Hispanic, American Indian, and missing/unknown.