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. Author manuscript; available in PMC: 2019 Sep 25.
Published in final edited form as: Ann Intern Med. 2018 Dec 11;170(1):22–30. doi: 10.7326/M18-1323

Figure 3. HR for breast cancer according to years since most recent birth, stratified by age at first birth (nulliparous or parous with age at first birth <25, 25–34, or 35–39 y).

Figure 3.

Nulliparous women are the reference group, and HRs are adjusted for attained age, study, and parity (0, 1, or ≥2 births). Dashed curves correspond to 95% CIs. Vertical lines represent the quadratic spline knots at 6.1, 13.3, 18.4, 23.3, and 29.3 y after birth. Likelihood ratio tests indicated a statistically significant interaction with age at first birth (P = 0.013). All analyses censor at age 55 y; therefore, only the first 15 y of follow-up are analyzed for women in the oldest age group (35–39 y) because they were aged ≥50 y after 15 y. Women who had their first birth before age 25 y had a peak HR for breast cancer of 1.06 (95% CI, 0.67 to 1.66) at <1 y after birth. For women who had their first births at age 25–34 y and 35–39 y, peak HRs were 1.25 (CI, 1.11 to 1.40) at 4.6 y and 1.40 (CI, 1.14 to 1.72) at 6.4 y, respectively, since most recent birth. HR = hazard ratio.