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

Age at baseline

   55–59

   60–64

   65–69

   70–74

Race/ethnicity

   White

   African-American

   Asian/Pacific Islander

   Other*

Education

   Less than H.S.

   Completed H.S.

   Beyond H.S.

   Unknown

Marital status

   Married or living as married

   Widowed

   Divorced or separated

   Never married/missing

Mother or sister with breast cancer

   No

   Yes

   Unknown

Age at menarche

   < 12

   12 – 13

   14 – 15

   ≥ 16

   Missing

Years of oral contraceptive use

   None

   < 1 year

   2 – 3 years

   4 – 5 years

   6 – 9 years

   ≥ 10 years

   Missing

Parity and age at first birth

   Nulliparous

   1–2

< 20 years

20–24 years

25–29 years

30–34 years

≥ 35 years

   3–4

< 20 years

20–24 years

25–29 years

30–34 years

≥ 35 years

   ≥ 5

< 20 years

20–24 years

25–29 years

30–34 years

≥ 35 years

   Missing or unknown

Menopause Type

   Natural

   Surgical

   Radiation or medications

   Missing

Age at Natural Menopause

   < 45 years

   45–49 years

   50–54 years

   55+ years

   Missing

Menopausal hormone therapy use

   None

   Former use

   Current use, ≤ 5 years

   Current use, > 5 years

   Missing

Body Mass Index (BMI; kg/m²)

   < 18.5

   18.5 – 24.9

   25 – 29

   30 – 34.9

   ≥ 35

   Missing