Table 2:

ORs and 95% CIs for select reproductive risk factors and breast cancer risk overall, stratified by women younger and older than 50 years among 1,122 cases and 2,096 controls

	All women						Women <50 years old (N = 564)						Women ≥50 years old (N = 558)
	OR	95% CI		p	p-trend	Controls	Cases	OR	95% CI		p	p-trend	Controls	Cases	OR	95% CI		p	p-trend	p-int (LRT)
Age at menarche (years)
<15	1·00					391	148	1·00					174	118	1·00
15	0·88	0·70	1·10	0·25		323	121	0·88	0·65	1·20	0·42		225	134	0·81	0·57	1·14	0·22
16	1·13	0·89	1·44	0·31		238	114	1·17	0·86	1·61	0·31		143	108	0·94	0·64	1·38	0·75
≥17	1·08	0·85	1·37	0·53	0·30	249	117	1·08	0·79	1·48	0·63	0·46	146	111	0·97	0·66	1·42	0·87	0·82	0·34
Parity
Nulliparous	1·00					209	73	1·00					19	34	1·00
1–2	1·04	0·72	1·51	0·83		406	204	0·93	0·58	1·50	0·78		122	115	0·59	0·28	1·24	0·16
3–4	0·80	0·55	1·15	0·23		429	187	0·79	0·49	1·27	0·32		254	176	0·41	0·20	0·84	0·01
≥5	0·73	0·50	1·07	0·10	0·005	246	99	0·70	0·42	1·18	0·18	0·06	403	230	0·40	0·20	0·83	0·01	0·01	0·02
Age at first birth (years)
<19	1·00					310	103	1·00					242	132	1·00
19–21	1·14	0·90	1·43	0·28		290	121	1·18	0·85	1·64	0·32		219	143	1·15	0·82	1·60	0·42
22–25	1·27	1·00	1·62	0·05		247	127	1·42	1·01	2·00	0·04		164	133	1·15	0·81	1·65	0·43
≥26	1·18	0·91	1·54	0·22	0·135	204	121	1·40	0·97	2·01	0·07	0·05	118	76	1·03	0·68	1·56	0·88	0·74	0·28
Median months breastfeeding per pregnancy (among parous women)
<13	1·00					189	89	1·00					163	150	1·00
13–18	0·85	0·68	1·06	0·16		416	182	0·98	0·71	1·36	0·92		272	158	0·74	0·54	1·03	0·07
≥19	0·84	0·67	1·05	0·12	0·159	434	184	1·04	0·75	1·44	0·82	0·77	308	160	0·71	0·51	0·98	0·04	0·05	0·01

Logistic regression models were adjusted for age, study site, body size, family history of breast cancer, menopausal status, age at menopause, and all reproductive factors listed above. P-values <0·05 were considered statistically significant. OR=odds ratio. CI=confidence interval, LRT=Likelihood ratio test for interaction term.