Table 1.

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

	Oral Contraceptive Use
	Never	Ever	<5 years ago	≥10 years duration
N	13,186	40,662	13,552	6,538
Age, mean	42.1	38.3	30.1	39.4
Age at menarche, mean	12.3	12.4	12.6	12.3
BMI* at age 18, mean	22.0	21.3	21.2	20.9
BMI, mean	28.8	27.8	27.3	27.2
Education, %
≤ 12 years	24.0	17.5	20.8	13.4
13–15	34.5	35.9	32.1	33.4
≥16	41.3	46.5	46.9	53.1
Family history of breast cancer, %	6.8	6.5	7.6	6.5
Parity, %
Nulliparous	42.1	33.4	38.5	41.5
1	17.2	22.5	25.3	25.3
2	19.0	23.8	14.3	21.1
≥3	21.4	20.1	21.8	12.1
Age at first birth, parous only, %
< 20	37.6	33.5	25.3	32.3
20–24	37.6	35.6	35.1	31.5
≥25	23.8	29.9	34.6	35.0
Premenopausal, %	74.6	75.8	80.8	77.2
Ever FH** use, %	14.8	16.7	9.7	16.9
Vigorous activity, %
None	34.1	31.5	29.3	31.1
< 5 hrs/wk	48.0	51.5	54.2	52.3
≥ 5 hrs/wk	13.0	13.2	12.0	13.6
Alcohol, %
<1 drink/wk	78.9	73.3	73.9	70.2
1–6 drinks/wk	14.9	20.2	19.5	22.9
≥ 7 drinks/wk	5.1	5.9	6.0	6.2

^*Body mass index

^**Female hormones