Table 4.

Relationship between age at menarche, breast feeding history, and risk of breast cancer subtypes^a

	Luminal A	Luminal B		Triple-negative		HER2-overexpressing
	n (%)	n (%)	OR (95% CI)	n (%)	OR (95% CI)	n (%)	OR (95% CI)
Age at menarche
<12	105 (19.8)	17 (21.5)	1.00 (ref)	169 (22.1)	1.00 (ref)	64 (21.6)	1.00 (ref)
12–13	308 (58.0)	49 (62.0)	0.95 (0.52–1.73)	439 (57.4)	0.90 (0.67–1.19)	167 (56.4)	0.88 (0.61–1.26)
≥14	118 (22.2)	13 (16.5)	0.64 (0.30–1.39)	157 (20.5)	0.83 (0.59–1.16)	65 (22.0)	0.88 (0.57–1.36)
Every 2-year			0.89 (0.65–1.22)		0.92 (0.79–1.06)		0.98 (0.81–1.18)
P_trend			0. 474		0.246		0.828
Ever breast fed a child^b
No	85 (21.3)	8 (14.0)	1.00 (ref)	144 (25.0)	1.00 (ref)	58 (23.7)	1.00 (ref)
Yes	314 (78.7)	49 (86.0)	1.30 (0.57–2.94)	432 (75.0)	0.77 (0.56–1.06)	187 (76.3)	0.80 (0.54–1.19)
Breast feeding duration (in months)^b
Never	85 (21.3)	8 (14.0)	1.00 (ref)	144 (25.0)	1.00 (ref)	58 (23.7)	1.00 (ref)
< 6	124 (31.1)	13 (22.8)	0.90 (0.35–2.33)	165 (28.6)	0.75 (0.52–1.07)	63 (25.7)	0.69 (0.44–1.10)
6–11	71 (17.8)	9 (15.8)	1.08 (0.38–3.01)	105 (18.2)	0.83 (0.55–1.25)	50 (20.4)	0.95 (0.57–1.57)
12–23	64 (16.0)	11 (19.3)	1.37 (0.50–3.72)	94 (16.3)	0.82 (0.53–1.26)	34 (13.9)	0.71 (0.41–1.22)
24–35	32 (8.0)	7 (12.3)	1.79 (0.58–5.53)	47 (8.2)	0.82 (0.48–1.40)	20 (8.2)	0.83 (0.43–1.62)
36+	23 (5.8)	9 (15.8)	3.47 (1.17–10.33)^c	21 (3.6)	0.51 (0.27–0.99)^c	20 (8.2)	1.18 (0.59–2.37)
Every 6 months			1.13 (1.03–1.23)^c		0.96 (0.91–1.02)		1.01 (0.95–1.08)
P_trend			0.008		0.151		0.716

↵^aAnalyses were restricted to Seattle cases who completed the interview. ORs were adjusted for age at diagnosis and year of diagnosis.
↵^bRestricted to parous women.
↵^cStatistically significant at P < 0.05.