Table 2.

Hazard ratios and 95% confidence intervals for premenopausal breast cancer by measures of body fatness throughout the life course, NHSII 1991–2009

	No. of person-years	No. of cases	Covariate-adjusted	Covariate-adjusted	Covariate-adjusted	Covariate-adjusted
HR (95% CI) Ia			HR (95% CI) IIb	HR (95% CI) IIIc	HR (95% CI) IVd
Birthweight
<2.5kg	89826	115	0.74 (0.58–0.94)	0.74 (0.58–0.95)	0.69 (0.54–0.89)	0.76 (0.59–0.98)
2.5–3.1kg	361094	462	0.75 (0.64–0.89)	0.75 (0.64–0.88)	0.71 (0.60–0.84)	0.76 (0.64–0.90)
3.2–3.8kg	545342	770	0.83 (0.71–0.96)	0.83 (0.71–0.96)	0.80 (0.69–0.93)	0.82 (0.71–0.96)
3.9+kg	137630	227	1.0	1.0	1.0	1.0
P for trend	1133893	1574	< 0.001	< 0.001	< 0.0001	0.003
Somatotype at age 5
Somatotype 1	258356	394	1.0	1.0	1.0	1.0
Somatotype 2	363726	510	0.93 (0.82–1.06)	0.93 (0.82–1.07)	0.94 (0.82–1.08)	0.94 (0.82–1.07)
Somatotype 3	273662	369	0.87 (0.75–1.00)	0.86 (0.75–1.00)	0.90 (0.78–1.05)	0.90 (0.77–1.04)
Somatotype 4	143438	195	0.85 (0.72–1.02)	0.85 (0.72–1.02)	0.93 (0.77–1.12)	0.92 (0.76–1.10)
Somatotype 5–9	73549	69	0.58 (0.45–0.75)	0.57 (0.44–0.73)	0.64 (0.49–0.84)	0.63 (0.48–0.82)
P for trend	1112731	1537	< 0.0001	< 0.0001	0.01	0.006
Somatotype at age 10
Somatotype 1	197453	310	1.0	1.0	1.0	1.0
Somatotype 2	349384	508	0.94 (0.82–1.09)	0.94 (0.81–1.08)	0.96 (0.83–1.11)	0.96 (0.83–1.11)
Sometotype 3	261139	371	0.91 (0.78–1.05)	0.90 (0.77–1.04)	0.95 (0.81–1.12)	0.95 (0.80–1.11)
Somatotype 4	180008	231	0.80 (0.68–0.95)	0.79 (0.66–0.94)	0.87 (0.72–1.05)	0.87 (0.72–1.05)
Somatotype 5–9	132173	134	0.62 (0.51–0.77)	0.61 (0.49–0.75)	0.70 (0.56–0.88)	0.69 (0.55–0.86)
P for trend	1120155	1554	< 0.0001	< 0.0001	0.003	0.002
BMI at age 18 (kg/m2)
< 18.5	162050	238	1.0	1.0	1.0	1.0
18.5–19.9	280418	434	1.05 (0.89–1.22)	1.09 (0.92–1.28)	1.10 (0.93–1.29)	1.12 (0.95–1.32)
20–22.4	409172	571	0.93 (0.80–1.09)	1.00 (0.85–1.17)	1.01 (0.85–1.20)	1.05 (0.88–1.24)
22.5–24.9	161060	205	0.85 (0.70–1.02)	0.94 (0.76–1.15)	0.96 (0.77–1.19)	1.00 (0.80–1.24)
25–27.4	63661	68	0.71 (0.54–0.92)	0.79 (0.59–1.06)	0.81 (0.60–1.09)	0.83 (0.62–1.13)
≥ 27.5	49487	42	0.58 (0.41–0.80)	0.67 (0.47–0.95)	0.69 (0.48–0.99)	0.72 (0.50–1.03)
P for trend	1125846	1558	< 0.0001	0.009	0.02	0.06
Current BMI (kg/m2) e
< 20	65864	109	1.0	1.0	1.0	1.0
20–22.4	209205	271	0.75 (0.60–0.94)	0.78 (0.62–0.99)	0.78 (0.62–0.99)	0.78 (0.62–0.99)
22.5–24.9	238623	367	0.86 (0.69–1.06)	0.91 (0.73–1.14)	0.91 (0.73–1.14)	0.91 (0.73–1.14)
25–27.4	197505	283	0.79 (0.63–0.99)	0.87 (0.69–1.10)	0.87 (0.69–1.10)	0.87 (0.69–1.09)
≥ 27.5	422696	544	0.70 (0.57–0.86)	0.84 (0.67–1.06)	0.84 (0.67–1.06)	0.83 (0.66–1.05)
P for trend	1133893	1574	0.003	0.62	0.62	0.53

^aAdjusted for age (continuous), premature birth (< 38 weeks, ≥ 38 weeks) and birthweight (except in the analysis of birthweight).

^bAdditionally adjusted for family history of breast cancer (yes, no), history of benign breast disease (yes, no), age at menarche (≤ 10 years, 11 years, 12 years, 13 years, 14 years, ≥ 15 years), interaction between parity (0, 1, 2, 3, 4, ≥ 5) and age at first birth (< 24 years, 25–30 years, > 30 years) with nulliparous women as reference, use of oral contraceptives (never, past and < 5 years, past and ≥ 5 years, current and < 5 years, current and 5–9 years, current and ≥ 10 years), alcohol consumption (never, < 7.5 g/day, 7.5–14 g/day, 15–29 g/day, ≥ 30 g/day), physical activity (< 3 mets/week, 4–8 mets/day, 9–17 mets/day, 18–26 mets/day, 27–41 mets/day, ≥ 42 mets/day) and body fatness factors earlier in life (somatotype or BMI). Since somatotype at age 5 and somatotype at age 10 were highly correlated (Spearman correlation coefficient = 0.81), these two factors were not adjusted for each other when one of the factors was assessed as the main exposure of interest.

^cAdditionally adjusted for body fatness factors (somatotype or BMI) later in life.

^dAdditionally adjusted for height (continuous in inches).

^eCurrent BMI is defined by the updated value assessed in the most recent previous biennial visit.