Table 1. Characteristics of Study Population.

	Premenopausal women
	Cases (n=945)		Controls (n=1,418)
	n	% 1	n	% 1	P 2
Study
San Francisco Bay Area Breast Cancer Study	430	46	666	47
4-Corners Breast Cancer Study	515	54	752	53
Estrogen receptor (ER) and progesterone receptor (PR) status
ER+PR+	575	61
ER+PR−	86	9
ER−PR+	37	4
ER−PR−	247	26
Age 3 (years)
<40	212	22	342	24
40-49	564	60	837	59
50-59	169	18	239	17
Ethnicity/English language acculturation
Hispanic - low acculturation	54	6	157	11	<0.01
Hispanic - moderate acculturation	196	21	339	24
Hispanic - high acculturation	247	26	269	19
Non-Hispanic White	448	47	653	46
Percent Indigenous American admixture 4
≤46	187	57	267	50	0.07
>46	140	43	267	50
Education
Some high school or less	148	16	313	22	<0.01
High school graduate	192	20	250	18
Some college or higher	605	64	855	60
Family history of breast cancer in first-degree relatives
No	795	84	1278	90	<0.01
Yes	150	16	140	10
Age at menarche (years)
<12	222	24	280	20	<0.01
12-13	497	53	713	50
≥14	226	24	425	30
Full-term pregnancies
Nulliparous	195	21	211	15	<0.01
1-2	474	50	619	44
3-4	227	24	469	33
≥5	49	5	119	8
Age at first full-term pregnancy (years)
Nulliparous	195	21	211	15
<25	376	40	669	47	<0.01
25-29	207	22	321	23
≥30	167	18	217	15
Lifetime breastfeeding (months)
Nulliparous	195	21	211	15	<0.01
0	198	21	238	17
1-12	294	31	463	33
13-24	119	13	250	18
≥25	139	15	256	18
Oral contraceptive use
Current	110	12	175	12	0.06
Former	648	69	886	62
Never	187	20	357	25
Alcohol consumption (g/day) 5
0	473	50	783	55	0.09
0.1-9.9	314	33	435	31
10.0-19.9	94	10	121	9
≥20.0	64	7	79	6

¹Percentages may not add up to 100% due to rounding.

²Mantel-Haenszel Chi-square test for difference between cases and controls.

³Age at diagnosis (cases) or selection into the study (controls).

⁴Among Hispanics only; based on the median in premenopausal controls.

⁵In the reference year.