Table 4.

Joint effects of parity and other reproductive variables on risk of ER+/PR+ and ER/PR breast cancer

Number of birthsER+/PR+ER/PR
CasesHR95% CICasesHR95% CI
Nulliparous1101.00Reference561.00Reference
Age at first birth
1<25460.78(0.55–1.11)291.16(0.73–1.83)
1≥25661.06(0.76–1.43)351.34(0.86–2.07)
≥2<251630.64(0.48–0.84)1351.48(1.04–2.09)
≥2≥25720.96(0.69–1.34)612.20(1.47–3.29)
Years since last birth
1<10241.95(1.22–3.11)121.27(0.67–2.41)
1≥10880.93(0.69–1.26)521.23(0.82–1.85)
≥2<10271.24(0.79–1.94)322.05(1.29–3.26)
≥2≥102020.86(0.64–1.16)1611.66(1.13–2.44)
Lactation
1Never730.76(0.55–1.05)441.00(0.65–1.52)
1Ever380.75(0.50–1.14)200.85(0.48–1.48)
≥2Never1190.64(0.48–0.85)1181.53(1.09–2.23)
≥2Ever1130.73(0.54–0.97)751.16(0.80–1.69)

NOTE: HRs were derived from models that adjusted for age, time, geographic region, age at menarche, age at menopause, use of menopausal hormone supplements, use of oral contraceptives, body mass index, physical activity, alcohol intake, cigarette smoking, family history of breast cancer, and each of the other reproductive factors.