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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 2. HR for breast cancer according to years since most recent birth for the joint effect of family history and time since most recent birth.

Figure 2.

Nulliparous women without a family history of breast cancer are the reference group, and HRs are adjusted for attained age, study, and continuous parity. 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 for models with and without interaction terms for time since most recent birth indicated a statistically significant (P = 0.044) interaction with family history of breast cancer. Compared with a common reference group of nulliparous women without such a history, women with a family history of breast cancer had an HR of 3.53 (CI, 2.91, 4.29) 4.9 y after first birth and did not cross over toward a protective effect for ≥30 y. As an approximation of the 95% CI around the crossover point, for women without a family history, the lower bound crosses at 22.9 y and the upper bound at 27.4 y. For women with a family history, the lower bound crosses at 34.2 y and the upper bound does not cross 1 during the 34.5 y of follow-up. HR = hazard ratio.