Table 3. Hazard ratios and 95% confidence intervals for the association of reproductive and menstrual factors with colorectal cancer incidence stratified by hormone therapy use among women enrolled in the WHI-OS.

	Hormone therapy use
	Never use	Ever use
Factor	Multivariable HRa (95% CI)	Multivariable HRa (95% CI)
Age at menarche (years)
⩽10	1.15 (0.71–1.85)	1.00 (0.58–1.74)
11–12	1.00 (referent)	1.00 (referent)
13–14	0.89 (0.71–1.11)	1.02 (0.81–1.28)
⩾15	0.72 (0.54–0.96)	0.92 (0.69–1.22)
P-trend	0.1	0.9
P-interaction	0.3
Age at first child's birth (years)
<20	1.00 (referent)	1.00 (referent)
20–29	0.99 (0.71–1.38)	0.97 (0.71–1.33)
⩾30	0.88 (0.56–1.38)	1.08 (0.69–1.67)
P-trend	0.79	0.84
P-interaction	0.56
Age at last child's birth (years)
<25	1.00 (referent)	1.00 (referent)
25–29	0.96 (0.65–1.42)	0.94 (0.67–1.30)
30–34	0.91 (0.62–1.35)	0.84 (0.60–1.18)
35–39	1.02 (0.68–1.53)	0.85 (0.57–1.25)
⩾40	0.81 (0.47–1.41)	1.08 (0.63–1.85)
P-trend	0.88	0.74
P-interaction	0.76
Pregnancy
Never	1.00 (referent)	1.00 (referent)
Ever	0.84 (0.63–1.11)	0.80 (0.59–1.07)
P-interaction	0.77
Parity
Nulliparous	1.00 (referent)	1.00 (referent)
1 child only	0.96 (0.66–1.39)	0.77 (0.51–1.16)
2 children	0.76 (0.56–1.03)	0.83 (0.61–1.13)
3 children	0.82 (0.61–1.11)	0.79 (0.58–1.08)
⩾4 children	0.76 (0.57–1.01)	0.94 (0.70–1.28)
P-trend	0.27	0.41
P-interaction	0.49
Induced abortion
Never	1.00 (referent)	1.00 (referent)
Ever	1.22 (0.85–1.75)	1.03 (0.71–1.49)
P-interaction	0.57
Breast fed for ⩾1 month
No	1.00 (referent)	1.00 (referent)
Yes	0.92 (0.76–1.10)	0.97 (0.81–1.17)
P-interaction	0.58
Age at menopause (years)
<40	1.00 (referent)	1.00 (referent)
40–44	0.93 (0.63–1.37)	0.81 (0.57–1.16)
45–49	0.89 (0.63–1.27)	1.06 (0.78–1.43)
50–54	0.80 (0.57–1.13)	0.99 (0.74–1.34)
⩾55	0.95 (0.64–1.42)	0.98 (0.67–1.41)
P-trend	0.64	0.6
P-interaction	0.48
Hysterectomy
No	1.00 (referent)	1.00 (referent)
Yes	1.23 (1.01–1.50)	0.90 (0.75–1.08)
P-interaction	0.02
Age at oophorectomy (years)
⩽34	1.00 (referent)	1.00 (referent)
35–39	1.99 (1.07–3.69)	1.23 (0.69–2.20)
40–44	1.54 (0.81–2.92)	1.13 (0.66–1.96)
45–49	1.38 (0.72–2.66)	0.90 (0.52–1.56)
⩾50	1.13 (0.62–2.07)	1.40 (0.86–2.30)
P-trend	0.19	0.27
P-interaction	0.33
Oral contraceptive use
Never	1.00 (referent)	1.00 (referent)
Ever	0.64 (0.51–0.81)	0.82 (0.67–1.01)
P-interaction	0.22
Duration of OC use (years)
1	1.00 (referent)	1.00 (referent)
2	0.79 (0.42–1.47)	1.38 (0.89–2.13)
3	1.58 (0.98–2.54)	0.94 (0.61–1.47)
4	0.71 (0.39–1.31)	1.03 (0.68–1.57)
P-trend	0.03	0.37
P-interaction	0.03

^aMultivariable model adjusted for age, family history of colorectal cancer, race/ethnicity, education level, a history of diabetes, smoking status and body mass index.