Table 4.

Total pack-years and duration of active smoking before the first term birth (FTB) in relation to breast cancer incidence overall and by menopausal status among parous women^a

Pack years and years of smoking before FTB		All women			Premenopausal		Postmenopausal
		Cases	P-yrsb	IRR (95% CI)	Cases	IRR (95 % CI)	Cases	IRR (95 % CI)
Never active or passive		98	57,022	1.00	40	1.00	43	1.00
Pack yrs	Yrs smoked before FTB
All	<5 yr	137	59,114	1.03 (0.78–1.35)	41	1.47 (0.92–2.33)	75	0.71 (0.48–1.05)
	≥5 yr	209	73,651	1.21 (0.94–1.56)	76	1.54 (1.03–2.30)	100	0.89 (0.61–1.29)
	Smoked after only	145	56,309	1.17 (0.89–1.55)	33	1.33 (0.81–2.19)	94	0.93 (0.63–1.37)
<20	<5 yr	81	41,536	0.93 (0.69–1.27)	32	1.51 (0.93–2.47)	38	0.61 (0.39–0.95)
	≥5 yr	146	55,334	1.20 (0.92–1.56)	57	1.43 (0.83–2.17)	62	0.89 (0.59–1.33)
	Smoked after only	106	42,888	1.20 (0.89–1.61)	30	1.48 (0.89–2.46)	65	0.98 (0.65–1.48)
≥20	<5 yr	52	15,175	1.31 (0.92–1.86)	9	1.64 (0.78–3.46)	35	0.94 (0.59–1.50)
	≥5 yr	56	15,913	1.25 (0.89–1.76)	16	2.01 (1.10–3.65)	34	0.88 (0.55–1.39)
	Smoked after only	31	10,984	1.09 (0.71–1.66)	1	—	24	0.82 (0.49–1.39)

^aIRRs adjusted for age, questionnaire cycle, BMI, years of education, family history of breast cancer, age at menarche, parity, oral contraceptive use, vigorous exercise, alcoholic beverage consumption, menopausal status, age at menopause, and menopausal female hormone use

^bPerson-years