Table 2.

Reproductive factors in relation to breast cancer subtypes^a

	ER⁺/PR⁺				ER⁺/PR⁻				ER⁻/PR⁻
	Cases	Person- years	HR	95% CI	Cases	Person- years	HR	95% CI	Cases	Person- years	HR	95% CI
Number of births
0	110	212,629	1.00	Reference	29	212,495	1.00	Reference	56	212,539	1.00	Reference
1	112	161,964	0.77	0.56–1.04	35	161,835	0.86	0.48–1.53	64	161,887	1.04	0.69–1.56
2	136	171,818	0.76	0.57–1.01	35	171,649	0.70	0.40–1.22	107	171,771	1.54	1.07–2.21
≥3	99	144,765	0.53	0.39–0.73	32	144,649	0.62	0.35–1.11	89	144,749	1.48	0.98–1.84
	P_trend = 0.0002				P_trend = 0.08				P_trend = 0.009
Age at first birth
<20	103	150,943	1.00	Reference	26	150,808	1.00	Reference	82	150,906	1.00	Reference
20–24	106	162,425	0.90	0.68–1.19	35	162,306	1.21	0.72–2.04	82	162,387	0.97	0.71–1.33
25–29	80	100,093	1.23	0.90–1.68	25	100,004	1.54	0.85–2.79	59	100,064	1.32	0.92–1.90
≥30	58	65,086	1.34	0.93–1.94	16	65,016	1.45	0.72–2.94	37	65,050	1.47	0.94–2.29
	P_trend = 0.05				P_trend = 0.18				P_trend = 0.04
Years since last birth
≥15	264	278,425	1.00	Reference	85	278,121	1.00	Reference	184	278,295	1.00	Reference
10–14	26	65,208	0.93	0.60–1.57	8	65,179	0.96	0.42–2.21	29	57,214	1.30	0.82–2.08
<10	51	128,913	1.69	1.08–2.63	9	128,121	0.89	0.34–2.34	44	128,901	1.61	0.97–2.68
	P_trend = 0.03				P_trend = 0.82				P_trend = 0.08
Lactation, parous women only
No	192	269,725	1.00	Reference	56	269,493	1.00	Reference	162	269,675	1.00	Reference
Yes	151	203,135	1.13	0.91–1.42	46	202,960	1.25	0.83–1.89	95	203,047	0.78	0.60–1.03
<6 mo	85	112,008	1.11	0.86–1.44	27	111,912	1.30	0.81–2.08	51	111,956	0.78	0.56–1.08
≥6 mo	66	91,127	1.17	0.87–1.57	19	91,048	1.18	0.68–2.06	44	91,091	0.79	0.56–1.13

NOTE: HRs were derived from models that adjusted for age, time, geographic region, age at menarche, age at menopause, use of menopausal hormone supplements, use of oral contraceptives, body mass index, physical activity, alcohol intake, cigarette smoking, family history of breast cancer, and each of the other reproductive factors.

^a18 breast cancer cases were classified as ER⁻/PR⁺ and were omitted from this table.