TABLE 2.

Odds ratios and 95% confidence intervals for invasive breast cancer according to early life factors, 2004−2006.

Characteristic	Cases (N=2,016)		Controls (N=1,960)		OR (95% CI)^*	OR (95% CI)^†
Characteristic	N	%^‡	N	%^‡	OR (95% CI)^*	OR (95% CI)^†
Maternal age^§
<20	142	7.0%	156	8.0%	0.81 (0.62, 1.04)	0.81 (0.62, 1.06)
20−24	595	29.5%	532	27.1%	1	1
25−29	524	26.0%	511	26.1%	0.92 (0.77, 1.09)	0.87 (0.73, 1.04)
30−34	344	17.1%	318	16.2%	0.97 (0.80, 1.18)	0.91 (0.73, 1.12)
≥35	299	14.8%	315	16.1%	0.85 (0.70, 1.03)	0.83 (0.66, 1.05)
5-year increase					1.00 (0.95, 1.05) p-value=0.9	0.99 (0.93, 1.05) p-value=0.7
Birth order^§
First-born	557	27.6%	514	26.2%	1	1
Second- or third-born	877	43.5%	833	42.5%	0.96 (0.83, 1.12)	0.99 (0.84, 1.17)
Fourth-born or higher	446	22.1%	491	25.1%	0.83 (0.69, 0.98)	0.88 (0.70, 1.10)
1-sibling increase					0.96 (0.93, 1.00) p-value=0.03	0.98 (0.94, 1.03) p-value=0.5
Breastfed in infancy
No	1014	50.3%	920	46.9%	1	1
Yes	634	31.4%	681	34.7%	0.87 (0.76, 1.01)	0.83 (0.72, 0.96)
Unknown/missing	368	18.3%	359	18.3%

Odds ratios adjusted for age.

^†

Odds ratios adjusted for age, age at menarche, age at first birth, parity, menopausal status, age at menopause, postmenopausal hormone use, family history of breast cancer, height, weight at age 20, weight gain, mammography use, and whether breastfed in infancy.

^‡

Due to missing values, some categories do not sum to 100%.

^§

In multivariable models, birth order and maternal age are adjusted for simultaneously when evaluating the effect of either variable.