Table 2.

Associations^a Between Reproductive Factors and Postmenopausal Breast Cancer Risk by Histological Type in the NIH-AARP Diet and Health Study Cohort, 1995–2006

	Person-Years	Histological Type															P for Heterogeneityb
		Ductal			Mucinous			Tubular			Lobular			Mixed Ductal-Lobular
		No. of Cases	HR	95% CI	No. of Cases	HR	95% CI	No. of Cases	HR	95% CI	No. of Cases	HR	95% CI	No. of Cases	HR	95% CI
Age at menarche, years
≤12	896,035	2,660	1	Reference	123	1	Reference	72	1	Reference	387	1	Reference	326	1	Reference	0.08
13–14	766,087	2,191	0.98	0.92, 1.04	77	0.63	0.46, 0.87	49	0.74	0.50, 1.07	374	1.09	0.94, 1.28	257	0.87	0.73, 1.04
≥15	174,041	469	0.92	0.83, 1.03	16	0.70	0.40, 1.22	11	0.73	0.38, 1.43	72	0.96	0.73, 1.26	54	0.89	0.65, 1.20
P-trend			0.15			0.01			0.13			0.68			0.16
Parityc
Nulliparous	256,371	881	1	Reference	35	1	Reference	29	1	Reference	118	1	Reference	110	1	Reference	0.10
Parous	1,561,723	4,364	0.78	0.72, 0.84	179	0.89	0.59, 1.34	103	0.55	0.35, 0.87	702	1.02	0.81, 1.27	520	0.84	0.66, 1.05
No. of children (among parous women)c
≥3	898,983	2,465	1	Reference	103	1	Reference	57	1	Reference	376	1	Reference	274	1	Reference	0.48
2	474,043	1,346	1.03	0.96, 1.11	52	0.97	0.67, 1.40	37	1.25	0.81, 1.92	218	1.13	0.94, 1.35	177	1.20	0.98, 1.47
1	188,697	553	1.12	1.02, 1.24	24	1.11	0.68, 1.81	9	0.90	0.44, 1.85	108	1.45	1.15, 1.83	69	1.18	0.89, 1.58
P-trend			0.03			0.80			0.79			<0.01			0.10
Age at first birth (among parous women), years
≤19	325,305	799	0.91	0.83, 0.99	38	1.18	0.78, 1.78	13	0.59	0.30, 1.13	92	0.79	0.61, 1.02	68	0.64	0.47, 0.87	<0.01
20–24	803,597	2,239	1	Reference	93	1	Reference	58	1	Reference	318	1	Reference	249	1	Reference
25–29	323,109	961	1.03	0.95, 1.12	40	1.17	0.78, 1.75	24	0.95	0.58, 1.58	207	1.58	1.31, 1.91	140	1.28	1.02, 1.60
≥30	104,584	350	1.15	1.01, 1.30	8	0.62	0.27, 1.42	8	1.08	0.51, 2.29	82	1.82	1.39, 2.37	62	1.87	1.39, 2.51
P-trend			<0.01			0.34			0.28			<0.01			<0.01
Type of menopause and age at menopause, years
Natural, <45	127,949	329	0.84	0.74, 0.96	12	1.19	0.62, 2.29	7	0.68	0.31, 1.51	58	0.94	0.69, 1.28	41	0.82	0.56, 1.19	0.01
Natural, 45–49	298,055	877	0.93	0.85, 1.02	30	1.21	0.74, 1.98	27	0.96	0.59, 1.56	133	0.91	0.73, 1.13	92	0.78	0.60, 1.01
Natural, 50–54	516,047	1,630	1	Reference	48	1	Reference	52	1	Reference	270	1	Reference	208	1	Reference
Natural, ≥55	114,664	465	1.27	1.13, 1.41	13	1.29	0.67, 2.47	15	1.36	0.76, 2.45	73	1.06	0.79, 1.40	60	1.32	0.97, 1.78
Bilateral oophorectomy	375,409	910	0.69	0.63, 0.75	64	1.53	0.98, 2.38	12	0.21	0.11, 0.42	140	0.63	0.50, 0.80	95	0.55	0.42, 0.73
Other surgical menopause	362,901	976	0.82	0.75, 0.90	43	1.28	0.81, 2.03	14	0.31	0.17, 0.58	144	0.74	0.59, 0.93	114	0.73	0.56, 0.94
Medically induced menopause or unknown age at natural menopause	48,209	147	0.92	0.76, 1.11	6	1.66	0.65, 4.27	5	0.78	0.31, 1.98	18	0.55	0.31, 0.99	29	1.32	0.86, 2.04

Abbreviations: CI, confidence interval; HR, hazard ratio; NIH, National Institutes of Health.

^a Estimates were adjusted for age at baseline, education, race, marital status, menopausal hormone use, oral contraceptive use, alcohol use, body mass index, frequency of vigorous physical activity, number of previous breast biopsies, family history of breast cancer (first-degree relative), and all other variables in the table, except where noted.

^b Two-sided P value from the Wald test, simultaneously comparing associations for ductal, mucinous, tubular, lobular, and mixed ductal-lobular types.

^c Not adjusted for age at first birth.