Table 2.

Multivariable analyses for associations of risk factors with invasive BC among women age <45 years, by ER status.^*

	All invasive (n=529)					ER- (n=151)				ER+ (n=219)
	person-years	cases	HR	95% CI		cases	HR	95% CI		cases	HR	95% CI
Age at menarche (yrs)
≤11	124,385	168	1.00	reference		53	1.00	reference		75	1.00	reference
12–13	221,987	275	0.86	0.71	1.04	72	0.73	0.51	1.04	110	0.79	0.58	1.06
≥14	73,990	83	0.73	0.56	0.96	25	0.70	0.43	1.13	32	0.67	0.44	1.01
Oral contraceptive use
Never	70,615	79	1.00	reference		19	1.00	reference		29	1.00	reference
Ever, ≥10 yrs ago	116,719	180	0.93	0.71	1.21	54	1.21	0.71	2.07	74	1.02	0.66	1.58
Ever, 5–10 yrs ago	63,872	82	1.07	0.78	1.46	19	1.03	0.54	1.97	34	1.12	0.67	1.85
Ever, within 5 yrs	171,110	188	1.25	0.95	1.64	59	1.53	0.90	2.62	82	1.40	0.90	2.16
Family history of breast cancer
No	397,064	456	1.00	reference		131	1.00	reference		190	1.00	reference
Yes	25,280	73	2.27	1.77	2.90	20	2.11	1.32	3.39	29	2.16	1.46	3.20
Parity
Nulliparous	163,006	156	1.11	0.80	1.55	43	1.30	0.69	2.47	68	0.90	0.54	1.48
1 birth	111,372	148	1.00	reference		40	1.00	reference		72	1.00	reference
2 births	90,252	135	1.07	0.84	1.37	40	1.30	0.82	2.05	51	0.80	0.54	1.16
≥3 births	54,927	90	1.21	0.89	1.64	28	1.71	0.98	2.99	28	0.69	0.41	1.14
Age at first birth (yrs)a
<20	64,929	86	1.00	reference		22	1.00	reference		35	1.00	reference
20–24	76,126	92	1.01	0.74	1.36	28	1.42	0.80	2.50	28	0.65	0.39	1.09
≥25	110,113	190	1.44	1.08	1.94	58	2.29	1.32	3.97	85	1.06	0.67	1.67
Years since last birtha
≥10 yrs	114,915	186	1.00	reference		54	1.00	reference		75	1.00	reference
<10 yrs	132,817	178	1.43	1.13	1.81	53	1.39	0.90	2.16	72	1.44	1.00	2.07
Lactationa
Never	133,107	199	1.00	reference		64	1.00	reference		70	1.00	reference
Ever	121,551	174	0.87	0.70	1.08	44	0.61	0.40	0.92	81	1.22	0.87	1.73
BMI at age 18 (kg/m2)
<20	152,177	216	1.00	reference		69	1.00	reference		87	1.00	reference
20–24	192,897	259	0.97	0.80	1.18	62	0.68	0.47	0.99	111	1.04	0.77	1.41
≥25	71,914	49	0.51	0.36	0.72	19	0.55	0.31	0.99	21	0.52	0.31	0.88
Current BMI (kg/m2)
<25	147,559	168	1.00	reference		40	1.00	reference		73	1.00	reference
25–29.9	125,668	174	1.03	0.83	1.29	57	1.47	0.97	2.25	63	0.87	0.62	1.24
≥30	145,616	183	1.04	0.81	1.33	53	1.24	0.76	2.01	83	1.10	0.76	1.60
Waist:hip ratio
Tertile 1 (<0.76)	113,945	123	1.00	reference		33	1.00	reference		55	1.00	reference
Tertile 2 (0.76 – 0.84)	102,018	114	1.00	0.77	1.29	28	0.89	0.54	1.48	53	1.00	0.68	1.47
Tertile 3 (>0.84)	101,901	141	1.30	1.01	1.68	50	1.64	1.04	2.59	45	0.88	0.58	1.33
Alcohol consumption (drinks)
<1/wk	300,559	372	1.00	reference		114	1.00	reference		151	1.00	reference
1–6/wk	104,727	130	0.96	0.78	1.18	29	0.69	0.46	1.05	56	0.94	0.69	1.29
≥7/wk	16,542	27	1.28	0.86	1.91	5	0.73	0.30	1.83	12	1.39	0.76	2.55
Vigorous exercise
None	155,744	211	1.00	reference		64	1.00	reference		86	1.00	reference
<5 hrs/week	215,857	258	0.96	0.80	1.16	73	0.97	0.69	1.37	110	0.93	0.28	3.02
≥5 hrs/week	47,631	55	1.00	0.74	1.35	13	0.87	0.47	1.59	23	1.00	0.75	1.33

^*Multivariable hazards ratios (HRs) adjusted for age, smoking history (never active or passive, passive only, never active/unknown passive, 1-<10 packyears, 10+ packyears), menopausal status (premenopausal, hysterectomy only, postmenopausal, missing), and all factors in Table 2.

^aAmong parous women only. Estimates for age at first birth adjusted for age, smoking history, menopausal status, and all factors in Table 2 except years since last birth; estimates for years since last birth adjusted for age, smoking history, menopausal status, and all factors in Table 2 except age at first birth; estimates for lactation adjusted for age, smoking history, menopausal status, and all factors in Table 2.