Table 2.

Risk of Breast Cancer in Relation to Reproductive History and Lifestyle Factors at Baseline Interview, Shanghai, China, 1989–2000

Risk Factor	Cases (n = 1,709)		Noncases (n = 4,780)		Adjusted for Age		Adjusted for Age and No. of Live Births
					HR	95% CI	HR	95% CI
	No.	%	No.	%
Age at first menstrual period, years
≤13	179	10.5	450	9.4	1.00	Referent	1.00	Referent
14	315	28.4	794	16.6	1.00	0.81, 1.25	1.02	0.82, 1.27
15	415	29.1	1,013	21.2	1.04	0.85, 1.28	1.05	0.86, 1.30
16	360	25.7	1,041	21.8	0.91	0.74, 1.12	0.93	0.76, 1.16
≥17	439	22.9	1,482	31.0	0.80	0.65, 0.99	0.84	0.68, 1.03
No. of live births
0	118	6.9	203	4.2	1.32	1.02, 1.70
1	695	40.7	1,644	34.4	1.00	Referent
2	367	31.4	802	16.8	1.02	0.85, 1.23
3	244	23.0	816	17.1	0.67	0.53, 0.84
4	150	8.8	628	13.1	0.52	0.40, 0.68
≥5	135	7.9	687	14.4	0.42	0.32, 0.56
Age at first live birth, years
Nulliparous	118	6.9	203	4.2	2.42	1.72, 3.42	1.14	0.93, 1.39
≤19	72	4.2	321	6.7	1.00	Referent	1.00	Referent
20–24	462	21.7	1,666	34.8	1.22	0.93, 1.60	1.09	0.82, 1.44
25–29	721	26.9	1,962	41.0	1.52	1.15, 2.00	1.18	0.88, 1.57
≥30	336	34.9	628	13.1	2.14	1.58, 2.88	1.61	1.18, 2.21
Duration of breast feeding, months
Nulliparous	118	6.9	203	4.2	1.41	1.07, 1.84	1.05	0.84, 1.30
Never breastfed	240	14.0	577	12.1	1.00	Referent	1.00	Referent
≤6	205	28.2	522	10.9	0.93	0.75, 1.16	0.9	0.72, 1.13
7–12	456	29.5	1,090	22.8	0.99	0.82, 1.19	0.96	0.80, 1.16
13–24	298	28.4	750	15.7	0.97	0.79, 1.20	0.91	0.72, 1.15
25–36	185	24.2	580	12.1	0.81	0.64, 1.02	0.94	0.72, 1.23
37–48	102	6.0	426	8.9	0.61	0.46, 0.80	0.77	0.56, 1.05
≥49	105	6.1	632	13.2	0.42	0.32, 0.55	0.56	0.40, 0.77
Cigarette smoking
No	1,663	97.4	4,607	96.4	1.00	Referent	1.00	Referent
Former user	8	0.5	29	0.6	0.88	0.40, 1.92	0.87	0.39, 1.94
Current user	37	2.2	144	3.0	0.82	0.57, 1.19	0.87	0.60, 1.26
Alcohol use
No	1,368	80.0	3,845	80.4	1.00	Referent	1.00	Referent
Yes	341	20	935	19.6	0.98	0.85, 1.13	0.99	0.86, 1.14

Abbreviations: CI, confidence interval; HR, hazard ratio.