Table 4.

BC risk associated with the combined effect of number of childbirths and breastfeeding duration in the Korea Breast Cancer Study, 2004 to 2012

No. of childbirths	Breastfeeding duration (mo)	Premenopausal women	Postmenopausal women
0	0	1.00 (reference)	1.00 (reference)
1	0	0.98 (0.70, 1.39)	0.65 (0.43, 0.99)
2	0	0.78 (0.69, 0.92)	0.63 (0.54, 0.73)
3+	0	0.89 (0.86, 0.94)1	0.47 (0.44, 0.50)1
1	1-12	0.65 (0.58, 0.74)	0.64 (0.55, 0.74)
1	13-24	0.69 (0.51, 0.60)	0.49 (0.42, 0.58)
2	1-12	0.52 (0.45, 0.61)	0.52 (0.43, 0.62)
3+	1-12	0.63 (0.59, 0.68)	0.70 (0.61, 0.80)
2	13-24	0.44 (0.39, 0.51)	0.36 (0.30, 0.44)
2	25+	0.36 (0.34, 0.39)1	0.27 (0.23, 0.32)1
3+	13-24	0.36 (0.35, 0.38)1	0.29 (0.25, 0.33)1
3+	25+	0.34 (0.32, 0.38)1	0.21 (0.18, 0.26)1

Values are presented as OR (95% CI).

The OR (95% CI) in all women was estimated in conditional logistic regression stratified by age and enrollment year and adjusted for family history of BC, age at menarche, age at first full-term pregnancy, duration of oral contraceptive use, BMI, and breastfeeding duration in the analyses of parity and number of childbirths; or adjusted for family history of BC, age at menarche, age at first full-term pregnancy, duration of oral contraceptive use, BMI, and number of childbirths in the analyses of breastfeeding and its duration. The calculated actual OR values in premenopausal and postmenopausal women was estimated using an unconditional logistic regression model stratified by age and year and adjusted for same covariates.

BC, breast cancer, OR, odds ratio; CI, confidence interval; BMI, body mass index.

¹p-value for the heterogeneity of two ORs (95% CIs) from 2 subgroups classified by menopausal status: <0.001 for [children=3+ and never breastfeeding] 0.005 for [children=3+ and breastfeeding=13-24 months]; 0.002 for [children=2 and breastfeeding=25+ months]; <0.001 for [children=3+ and breastfeeding=25+ months].