Introduction 1 4 5 7 8 9 11 12 16 17 18 17 18 19 The aim of this study was to evaluate the relation between the “lifetime cumulative number of menstrual cycles” and the reproductive characteristics that contribute to this estimate on one side, and plasma concentrations of sex hormones on the other side, in order to elucidate the mechanism by which reproductive risk factors could influence hormone-related cancers. Study design and methods 20 21 21 n n n n 17 22 Bounded levels of estrogens and androgens were measured, levels of estrone (E1), estradiol (E2), androstenedione, testosterone, sex hormone binding globulin (SHBG) and dehydroepiandrostenedione sulfate (DHEAS) were measured in plasma using commercially available double-antibody radioimmunoassay (RIA) kits (Diagnostic System Laboratories Inc., Webster, TX, USA). The following kits were used: E1: DSL-8700, E2: DSL-39100, androstenedione: DSL-4200, TST: DSL-4100 SHBG: DSL-6300 and DHEAS: DSL-2700. The intra-assay coefficients of variation were 5.6%, 3.9%, 4.3%, 7.7%, 3.0% and 5.2% respectively. The inter-assay coefficients of variation were 11.1%, 4.1%, 6.3%, 8.1%, 4.0% and 5.3% respectively. All hormone assays were performed by the laboratory of the Hormones and Cancer group at the International Agency for Research on Caner (IARC). Although technically SHBG is not a hormone, for reasons of convenience it will be referred as such. We calculated the free-androgen index (FAI) dividing testosterone by SHBG and multiplying by 100, the SHGB/E2 index was calculated dividing SHBG by E2. Data analysis Means with their standard deviation (SD) values, or frequencies of baseline characteristics were calculated. Concentrations of all hormones were logarithmically transformed to produce approximately normal distributions. To make the results easily interpretable, we transformed the mean and its 95% confidence interval (CI) back to their original scale, resulting in geometric means and 95% CIs. Geometric mean levels and 95% CIs were calculated for each hormone (SHBG, E1, E2, androstenedione, testosterone, DHEAS), SHBG/estradiol index (SHBG/E2) and free androgen index (FAI). 23 To test the significance of the models’ linearity, median values in each category for every variable were computed accordingly. Subsequently these values were included as a continuous variable in a linear regression model to test for linear trends. Because there is no clear natural hierarchy, the “always irregular” category was not included when testing for trend for the “lifetime number of menstrual cycles” and the “number of menstrual cycles before a FFTP” variables. Additional mutual adjustments for reproductive characteristics did not show any difference in the observed trends. The Statistical Package for Social Sciences for WINDOWS, version 14.0 (Chicago, IL), was used for all statistical analysis, and all tests were two sided. Results 1 2 17 2 Table 1 n   Mean (SD) General characteristics Age at enrollment 60.2 (5.5) BMI 26.2 (4.2) Waist Hip ratio 0.797 (0.59) Length (cm) 163.7 (6.1) Weight (Kg) 70.1 (11.9) Educational level (%)  - Primary education 263 (30.6) - Low technical, secondary 384 (44.6) - High technical, secondary 98 (11.4) - Academic 115 (13.4) Ever married (%) 805 (93.6) Cigarette smoking habits (%)  - Never smoked 402 (46.7) - Past smoker 275 (32) - Current smoker 183 (21.3) Mother or sister with BC (%) 123 (14.3) Reproduction characteristics Age at menarche 13.6 (1.6) Nulliparous (%) 113 (13.1) a 25.46 (3.8) a 2.9 (1.4) Age at menopause 49.6 (4.0) Years since menopause 10.6 (6.0) Oral contraceptive use (%) 384 (44.6) b 6 HRT use (%) 66 (7.7) c 1 Lifetime number of menstrual cycles  n 165 (21.0)   ≤415 177 (22.5)   416–453 171 (21.8)   454–490 161 (20.5)   ≥491 112 (14.2) d  Number of menstrual cycles before  n    ≤118 167 (25.5)   119–147 160 (23.4)   148–185 118 (17.3)   ≥186 143 (20.9) d 95 (13.9) a b c d Table 2 Geometric mean serum sex hormone and SHBG levels Hormone Mean, (95% CI) SHBG μg/ml 18.1 (17.2–19.2) Estrone pg/ml 15.2 (14.48–16.0) Estradiol pg/ml 8.5 (8.2–8.8) Androstenedione ng/ml 0.47 (0.45–0.49) Testosterone total ng/ml 0.26 (0.25–0.27) DHEAS ng/ml 455.1 (432.9–478.4) SHBG- Estradiol index (μg/pg) 0.47 (0.44–0.50) Free androgen index [(Testosterone /SHBG*100 (ng/μg*100)] 4.6 (4.4–5.0) 3 P P Table 3 Hormones unadjusted means (95%CI) according to patient characteristics   SHBG (μg/ml) Estrone (pg/ml) Estroidal (pg/ml) Androstenedione (ng/ml) Testosterone (ng/ml) DHEAS (ng/ml) SHBG/E2-index FA-index Lifetime number of menstrual cycles     <=415 19.8 (17.3–22.5) 15.6 (13.9–17.4) 8.5 (7.8–9.2) 0.56 (0.50–0.64) 0.29 (0.26–0.31) 515.3 (458.6–579.1) 0.431 (0.366–0.509) 4.7 (4.05–5.46)     416–453 19.5 (17.1–22.1) 13.5 (12.1–15.0) 8.5 (7.8–9.2) 0.45 (0.39–0.50) 0.25 (0.23–0.27) 448.1 (400.3–501.5) 0.435 (0.371-0.511) 4.3 (3.73–4.97)     454–490 17.4 (15.3–19.8) 16.1 (14.4–17.9) 8.8 (8.1–9.5) 0.43 (0.39–0.49) 0.25 (0.23–0.27) 436.4 (389.1–489.3) 0.504 (0.428–0.593) 4.7 (4.09–5.49)      >= 491 15.9 (13.9–18.2) 15.9 (14.2–17.9) 8.7 (7.9–9.4) 0.44 (0.39–0.50) 0.25 (0.23–0.27) 434.8 (386.4–489.3) 0.542 (0.458–0.641) 5.1 (4.36–5.89)     Always irregular § 16.6 (14.1–19.5) 14.9 (13.04–17.1) 7.9 (7.1–8.7) 0.49 (0.40–0.54) 0.26 (0.23–0.28) 425.4 (69.2–490.2) 0.476 (0.389–0.582) 4.9 (4.07–5.85)     P 0.15 0.455 0.648 0.003 0.052 0.035 0.031 0.406 Number of menstrual cycles before a FFTP*     <= 118 19.5 (17.1–22.2) 14.8 (13.2–16.6) 8.3 (7.6–8.9) 0.48 (0.42–0.54) 0.27 (0.24–0.24) 472.1 (420.1–530.6) .426 (.362-.500) 4.5 (3.86–5.15)     119–147 18.2 (15.9–20.8) 14.9 (13.2–16.7) 8.6 (7.9–9.3) 0.48 (0.42–0.54) 0.26 (0.23–0.23) 488.8 (433.8–550.7) .471 (.399-.555) 4.6 (3.96–5.32)     148–185 17.3 (14.8–20.1) 15.9 (13.9–18.2) 9.2 (8.3–10.0) 0.43 (0.37–0.50) 0.26 (0.23–0.23) 425.1 (369.9–488.4) .529 (.437-.641) 4.9 (4.17–5.89)     >= 186 18.8 (16.3–21.6) 14.5 (12.8–16.4) 8.3 (7.6–8.9) 0.45 (0.39–0.51) 0.25 (0.22–0.22) 419.4 (369.6–475.7) 0.440 (0.370–0.524) 4.4 (3.76–5.15)     Always irregular § 15.5 (13.1–18.4) 14.8 (12.7–17.2) 8.0 (7.2–8.8) 0.48 (0.40–0.56) 0.27 (0.24–0.24) 440.3 (377.1–513.9) 0.515 (0.416-.638) 5.5 (4.51–6.62)     P 0.574 0.992 0.933 0.349 0.333 0.087 0.565 0.999 Age at intake     <= 54 18.8 (16.5–21.5) 15.9 (14.3–17.8) 9.4 (8.7–10.2) 0.53 (0.47–0.60) 0.28 (0.26–0.31) 611.5 (547.5–682.9) 0.499 (0.424–0.587) 4.9 (4.19–5.62)     55–59 18.9 (16.9–21.2) 15.45 (14.1–16.9) 8.6 (8.0–9.2) 0.46 (0.42–0.51) 0.25 (0.23–0.27) 493.4 (448.6–542.7) 0.453 (0.394–0.522) 4.5 (3.92–5.04)     60–64 17.3 (15.6–19.3) 14.3 (13.0–15.6) 8.3 (7.8–8.8) 0.47 (0.43–0.52) 0.26 (0.24–0.28) 424.3 (387.5–464.5) 0.480 (0.419–0.548) 4.8 (4.26–5.42)     >= 65 17.7 (15.8–19.8) 15.4 (14.0–16.9) 7.9 (7.4–8.5) 0.44 (0.39–0.49) 0.25 (0.23–0.27) 363.9 (330.9–400.4) 0.450 (0.391–0.518) 4.6 (4.04–5.20)     P 0.303 0.449 0.002 0.041 0.106 <0.0001 0.499 0.834 Age at menarche     <= 11 18.9 (15.3–23.3) 15.2 (12.7–18.2) 8.0 (7.0–9.1) 0.47 (0.39–0.57) 0.28 (0.24–0.33) 437.1 (363.9–525.2) 0.423 (0.325–0.550) 4.9 (3.93–6.29)     12 15.9 (14.1–18.1) 14.5 (13.0–16.1) 8.8 (8.1–9.4) 0.47 (0.42–0.52) 0.26 (0.24–0.28) 439.4 (393.6–490.5) 0.549 (0.469–0.643) 4.2 (4.51–5.99)     13 17.5 (15.6–19.7) 15.9 (14.4–17.5) 8.3 (7.7–8.8) 0.47 (0.42–0.52) 0.26 (0.24–0.28) 461.7 (417.2–510.9) 0.472 (0.408–0.546) 4.7 (4.10–5.32)     14 18.8 (16.6–21.3) 15.1 (13.5–16.7) 8.6 (8.0–9.3) 0.44 (0.38–0.48) 0.24 (0.22–0.26) 447.9 (401.7–499.4) 0.458 (0.392–0.536) 4.2 (3.69–4.88)     15 20.8 (17.7–24.5) 15.9 (13.9–18.2) 8.5 (7.7–9.3) 0.49 (0.42–0.57) 0.28 (0.25–0.31) 490.6 (426.2–564.7) 0.408 (0.334–0.499) 4.4 (3.70–5.32)     >= 16 18.8 (16.1–21.9) 14.6 (12.7–16.6) 8.5 (7.7–9.3) 0.53 (0.46–0.61) 0.27 (0.24–0.29) 457.1 (399.0–523.5) 0.452 (0.372–0.549) 4.6 (3.86–5.47)     P 0.069 0.943 0.805 0.262 0.906 0.414 0.182 0.147 Reproductive status     Parous 17.5 (14.9–20.4) 16.9 (14.8–19.4) 8.8 (8.02–9.70) 0.52 (0.45–0.60) 0.26 (0.23–0.29) 461.5 (402.0–529.7) 0.506 (0.415–0.616) 4.8 (4.0–5.7)     Nulliparous 18.2 (17.2–19.4 14.9 (14.2–15.7) 8.5 (8.13 8.76) 0.47 (0.44–0.49) 0.26 (0.25–0.27) 454.1 (430.4–479.1) 0.463 (0.429–0.500) 4.7 (4.3–4.9)     P 0.608 0.083 0.407 0.168 0.853 0.831 0.418 0.770 Age at FFTP*     <= 21 19.5 (16.5–22.9) 15.7 (13.6–18.1) 8.5 (7.6–9.3) 0.48 (0.41–0.56) 0.26 (0.23–0.29) 481.3 (416.4–556.4) 0.434 (0.355–0.531) 4.4 (3.63–5.23)     22–24 16.2 (14.3–18.2) 14.7 (13.3–16.3) 8.8 (8.2–9.4) 0.47 (0.42–0.52) 0.27 (0.25–0.29) 468.1 (421.1–520.3) 0.544 (0.470–0.631) 5.2 (4.57–5.96)     25–27 18.5 (16.5–20.3) 15.2 (13.8–16.6) 8.2 (7.6–8.7) 0.47 (0.42–0.52) 0.26 (0.24–0.27) 454.9 (414.8–498.8) 0.445 (0.391–0.506) 4.6 (4.12–5.19)     >= 28 19.7 (17.4–22.2) 14.5 (13.06–16.6) 8.5 (7.8–9.1) 0.45 (0.40–0.50) 0.25 (0.23–0.27) 425.6 (382.4–473.6) 0.429 (0.370–0.498) 4.3 (3.75–4.90)     P 0.296 0.497 0.545 0.414 0.338 0.138 0.258 0.337 Number of liveborn     0 17.4 (14.9–20.4) 16.9 (14.8–19.3) 8.8 (8.0–9.7) 0.52 (0.45–0.60) 0.26 (0.23–0.29) 461.5 (401.9–529.7) 0.506 (0.415–0.616) 4.8 (4.00–5.71)     1 17.5 (14.1–21.8) 15.7 (13.1–18.9) 7.9 (7.0–9.1) 0.41 (0.33–0.50) 0.24 (0.21–0.28) 424.9 (351.1–514.3) .456 (.346-.600) 4.5 (3.48–5.69)     2 185 (16.7–20.3) 14.8 (13.6–16.1) 8.8 (8.2–9.2) 0.48 (0.43–0.52) 0.26 (0.24–0.28) 474.9 (436.4–516.8) 0.473 (0.419–0.534) 4.7 (4.21–5.23)     3 18.2 (16.2–20.5) 14.9 (13.4–16.4) 8.4 (7.7–8.9) 0.45 (0.40–0.50) 0.26 (0.24–0.28) 459.1 (414.4–508.6) 0.458 (0.395–0.531) 4.6 (4.00–5.21)     >= 4 18.1 (15.9–20.4) 15.0 (13.5–16.6) 8.2 (7.6–8.8) 0.48 (0.43–0.54) 0.27 (0.24–0.29) 425.7 (381.8–474.6) 0.455 (0.389–0.532) 4.7 (4.10–5.43)     P 0.770 0.174 0.289 0.624 0.668 0.432 0.437 0.946 BMI     <= 24.9 25.8 (23.7–28.2) 13.3 (12.3–14.4) 7.5 (7.1–7.9) 0.45 (0.41–0.49) 0.23 (0.22–0.25) 459.5 (459.5–498.5) 0.291 (0.262–0.323) 3.1 (2.81–3.41)     25–29.9 16.8 (15.5–18.2) 15.7 (14.6–16.8) 8.6 (8.1–9.0) 0.47 (0.44–0.51) 0.27 (0.25–0.28) 449.1 (449.1–484.2) 0.509 (0.461-0.561) 5.1 (4.70–5.63)     30–34.9 10.5 (9.1–12.1) 17.7 (15.5–20.1) 10.0 (9.1–10.9) 0.51 (0.44–0.58) 0.31 (0.28–0.34) 467.4 (467.4–535.0) 0.957 (0.803–1.14) 8.5 (7.24–9.98)     >= 35 9.9 (7.6–12.7) 21.5 (17.0–27.0) 13.5 (11.6–15.8) 0.56 (0.43–0.72) 0.31 (0.26–0.37) 439.8 (439.8–559.9) 1.37 (1.00–1.87) 9.0 (6.77–12.03)     P <0.0001 <0.0001 <0.0001 0.052 <0.0001 0.959 <0.0001 <0.0001 OC use ever     yes 17.6 (16.2–19.2) 15.5 (14.4–16.6) 8.4 (8.0–8.9) 0.47 (0.43–0.51) 0.26 (0.25–0.28) 494.7 (459.4–532.7) 0.480 (0.431–0.533) 4.9 (4.4–5.3)     no 18.6 (17.2–20.1) 14.9 (14.1–16.0) 8.5 (8.1–8.9) 0.47 (0.44–0.51) 0.26 (0.24–0.27) 424.8 (397.1–454.3) 0.460 (0.417–0.507) 4.5 (4.1–4.9)     P 0.354 0.553 0.727 0.938 0.625 0.003 0.564 0.258 HRT use ever     yes 17.9 (14.5–21.9) 18.5 (15.5–22.0) 11.9 (10.5–13.4) 0.52 (0.4–0.6) 0.26 (0.2–0.3) 488.8 (408.1–585.5) 0.667 (0.515–0.863) 4.6 (3.6–5.8)     no 18.2 (17.1–19.3) 14.9 (14.2–15.7) 8.3 (7.9–8.5) 0.47 (0.4–0.5) 0.26 (0.2–0.3) 452.4 (429.4–476.5) .455 (.422-.490) 4.7 (4.4- 4.9)     P 0.875 0.021  <0.0005 0.299 0.848 0.418 0.005 0.870 Age at menopause     <= 47 18.4 (16.33–20.6) 14.7 (13.3–16.2) 8.5 (7.9–9.1) 0.51 (0.46–0.57) 0.28 (0.26–0.30) 471.3 (425.7–521.9) 0.465 (0.402–0.539) 4.9 (4.3–5.6)     48–50 19.8 (17.94–21.8) 15.3 (14.1–16.6) 8.6 (8.1–9.1) 0.49 (0.44–0.53) 0.26 (0.24–0.27) 485.1 (445.4–528.4) 0.433 (0.383–0.490) 4.3 (3.8–4.8)     51–52 17.5 (15.49–19.8) 16.1 (14.5–17.9) 8.3 (7.7–8.9) 0.47 (0.42–0.52) 0.26 (0.24–0.28) 460.4 (413.4–512.7) 0.476 (0.408–0.556) 4.8 (4.2–5.5)     >= 53 16.0 (14.15–18.2) 0.159 14.6 (13.1–16.2) 8.5 (7.8–9.1) 0.41 (0.37–0.46) 0.24 (0.22–0.26) 389.2 (348.9–434.1) 0.527 (0.450-0.616) 4.9 (4.3–5.7)     P  0.702 0.739 0.014 0.017 0.039 0.335 0.950 Time since menopause (years)     <6 18.7 (16.7–20.9) 16.0 (14.5–17.6) 9.2 (8.6–9.9) 0.51 (0.46–0.57) 0.27 (0.25–0.29) 567.5 (514.6–625.9) 0.493 (0.427–0.568) 4.7 (4.14–5.35)     10-Jun 17.9 (15.9–20.0) 14.4 (13.1–15.9) 8.6 (8.1–9.3) 0.46 (0.42–0.51) 0.25 (0.23–0.27) 450.6 (408.0–497.7) 0.484 (0.419–0.559) 4.6 (3.99–5.19)     15-Nov 17.9 (15.9–20.1) 15.5 (13.9–17.1) 8.2 (7.6–8.8) 0.45 (0.40–0.50) 0.27 (0.24–0.29) 432.7 (390.7–479.2) 0.457 (0.394–0.530) 4.7 (4.13–5.39)     >= 16 18.1 (16.2–20.2) 14.9 (13.6–16.4) 8.1 (7.5–8.6) 0.47 (0.42–0.52) 0.26 (0.24–0.28) 390.1 (354.9–428.9) 0.443 (0.386–0.509) 4.7 (4.13–5.29)     P 0.725 0.489 0.003 0.233 0.828 <0.0001 0.246 0.960 * Among parous participants § The always irregular category was not included when testing for trend 4 P P P P P P p p Table 4 †   SHBG (μg/ml) Estrone (pg/ml) Estradiol (pg/ml) Androstenedione (ng/ml) Testosterone (ng/ml) DHEAS (ng/ml) SBHG/E2 -INDEX FA-INDEX Lifetime number of menstrual cycles     <= 415 19.50 (16.86–22.46) 15.75 (13.83–17.94) 8.62 (7.89–9.43) 0.571 (0.495–0.659) 0.285 (0.256–0.317) 519.57(454.41–594.67) 0.444 (0.373–0.528) 4.76 (4.06–5.59)     416–453 19.79 (17.51–22.35) 13.28 (11.88–14.83) 8.39 (7.78–9.04) 0.448 (0.396–0.506) 0.253 (0.231–0.278) 436.16 (388.77–488.82) 0.424 (0.365–0.492) 4.27 (3.72–4.89)     454–490 16.99 (14.98–19.26) 16.26 (14.51–18.23) 8.72 (8.06–9.41) 0.434 (0.383–0.493) 0.251 (0.228–0.276) 427.96 (380.31–481.06) 0.513 (0.440–0.597) 4.76 (4.14–5.48)     >= 491 15.94 (13.82–18.39) 16.02 (14.07–18.24) 8.53 (7.81–9.32) 0.443 (0.384–0.511) 0.257 (0.231–0.287) 447.64 (391.51–511.83) 0.535 (0.450–0.636) 5.15 (4.39–6.04)     Always irregular § 16.48 (13.87–19.57) 15.21 (13.01–17.78) 8.04 (7.24–8.94) 0.462 (0.389–0.548) 0.255 (0.224–0.290) 427.09 (363.58–501.69) 0.488 (0.397–0.601) 4.93 (4.07–5.96)     P 0.013 0.474 0.842 0.089 0.446 0.349 0.050 0.192 Number of menstrual cycles before a FFTP*     <= 118 19.83 (17.51–22.47) 15.15 (13.49–17.01) 8.22 (7.61–8.87) 0.474 (0.416–0.540) 0.265 (0.241–0.292) 468.25 (415.30–527.42) 0.414 (0.356–0.482) 4.37 (3.81–5.02)     119–147 18.41 (16.18–20.95) 14.47 (12.83–16.33) 8.38 (7.75–9.07) 0.480 (0.419–0.550) 0.253 (0.229–0.280) 475.80 (420.31–538.61) 0.455 (0.389–0.533) 4.53 (3.92–5.22)     148–185 16.09 (13.81–18.76) 15.66 (13.57–18.06) 8.85 (8.05–9.72) 0.420 (0.358–0.493) 0.258 (0.229–0.290) 402.22 (347.23–465.91) 0.549 (0.456–0.662) 5.17 (4.36–6.13)     >= 186 18.43 (16.13–21.05) 14.86 (13.13–16.83) 8.54 (7.88–9.27) 0.461 (0.401–0.529) 0.253 (0.228–0.280) 445.86 (392.29–506.74) 0.463 (0.394–0.545) 4.54 (3.92–5.27)     Always irregular § 15.52 (12.92–18.65) 14.81 (12.49–17.57) 8.04 (7.19–8.99) 0.467 (0.386–0.566) 0.267 (0.232–0.307) 438.34 (367.97–522.69) 0.518 (0.415–0.648) 5.39 (4.397–6.60)     P 0.282 0.929 0.416 0.605 0.573 0.423 0.196 0.522 Age at menarche     <= 11 19.26 (15.72–23.67) 14.67 (12.23–17.58) 7.56 (6.67–8.55) 0.440 (0.360–0.538) 0.272 (0.234–0.316) 424.54 (352.13–511.83) 0.392 (0.307–0.502) 4.71 (3.75–5.90)     12 18.10 (16.04–20.43) 13.98 (12.55–15.58) 8.53 (7.93–9.19) 0.454 (0.403–0.512) 0.249 (0.228–0.272) 423.27 (378.42–472.95) 0.471 (0.407–0.546) 4.51 (3.94–5.16)     13 16.35 (14.58–18.32) 15.52 (14.01–17.18) 8.17 (7.63–8.77) 0.458 (0.409–0.513) 0.254 (0.233–0.276) 444.52 (400.21–493.74) 0.500 (0.436–0.574) 4.87 (4.29–5.53)     14 17.79 (15.78–20.06) 15.41 (13.85–17.15) 8.58 (7.98–9.23) 0.440 (0.391–0.496) 0.248 (0.227–0.271) 448.99 (402.22–501.69) 0.482 (0.417–0.558) 4.58 (4.01–5.23)     15 19.73 (16.91–23.01) 17.06 (14.86–19.57) 9.02 (8.21–9.90) 0.518 (0.444–0.603) 0.286 (0.255–0.320) 516.98 (448.54–595.26) 0.457 (0.379–0.550) 4.78 (4.03–5.67)     >= 16 18.58 (15.99–21.58) 15.06 (13.16–17.22) 8.78 (8.02–9.63) 0.553 (0.476–0.641) 0.276 (0.247–0.308) 494.23 (430.09–567.36) 0.473 (0.395–0.567) 4.79 (4.05–5.66)     P 0.577 0.181 0.058 0.027 0.194 0.022 0.621 0.739 Ever pregnant     yes 18.17 (17.13–19.28) 14.94 (14.17–15.75) 8.41 (8.12–8.72) 0.463 (0.437–0.491) 0.258 (0.247–0.269) 452.59 (428.80–478.18) 0.463 (431–497) 4.62 (4.33–4.94)     no 16.56 (14.19–19.34) 17.12 (4.91–19.65) 8.88 (8.08–9.76) 0.528 (0.453–0.616) 0.273 (0.244–0.306) 466.85 (405.05–538.08) 0.536 (0.445–0.646) 5.21 (4.39–6.18)     P 0.273 0.072 0.299 0.119 0.802 0.693 0.152 0.204 Age at FFTP*     <= 21 19.89 (16.98–23.29) 15.53 (13.45–17.94) 8.26 (7.50–9.08) 0.454 (0.386–0.534) 0.248 (0.220–0.280) 451.24 (388.77–523.74) 4.05 (3.39–4.82) 0.415 (0.343–0.503)     22–24 17.46 (15.52–19.63) 14.41 (12.96–16.04) 8.46 (7.88–9.07) 0.466 (0.413–0.526) 0.264 (0.242–0.289) 464.05 (415.71–518.53) 4.84 (4.26–5.52) 0.484 (0.420–0.558)     25–27 17.32 (15.67–19.16) 15.01 (13.69–16.44) 8.06 (7.58–8.57) 0.463 (0.417–0.513) 0.257 (0.238–0.277) 445.41 (405.05–489.80) 4.84 (4.33–5.41) 0.465 (0.412–0.526)     >= 28 18.67 (16.63–20.99) 15.07 (13.56–16.76) 8.84 (8.24–9.48) 0.466 (0.414–0.525) 0.258 (0.237–0.282) 452.59 (405.45–504.72) 4.59 (4.04–5.22) 0.473 (0.411–0.545)     P 0.741 0.988 0.389 0.858 0.823 0.850 0.483 0.444 Number of liveborn     0 16.56 (14.18–19.32) 17.12 (14.91–19.67) 8.87 (8.08–9.76) 0.527 (0.452–0.615) 0.273 (0.244–0.306) 466.85 (404.64–538.08) 0.536 (0.445–0.646) 5.21 (4.39–6.18)     1 16.14 (13.05–19.94) 16.39 (13.56–19.81) 8.33 (7.32–9.49) 0.411 (0.333–0.508) 0.237 (0.203–0.277) 438.78 (361.04–533.79) 0.517 (0.400–0.667) 4.69 (3.71–5.95)     2 17.57 (15.99–19.32) 14.88 (13.68–16.20) 8.71 (8.22–9.23) 0.471 (0.429–0.517) 0.257 (0.240–0.276) 456.69 (418.63–498.19) 0.496 (0.443–0.555) 4.85 (4.37–5.39)     3 18.05 (16.13–20.19) 14.73 (13.33–16.29) 8.28 (7.73–8.86) 0.448 (0.401–0.502) 0.259 (0.239–0.282) 453.05 (408.71–502.70) 0.459 (0.401–0.525) 4.62 (4.07–5.23)     >= 4 20.18 (17.85–22.83) 14.83 (13.28–16.56) 8.12 (7.53–8.76) 0.486 (0.430–0.550) 0.263 (0.240–0.288) 449.89 (401.82–504.21) 0.402 (0.346–0.467) 4.23 (3.69- 4.85)     P 0.030 0.080 0.117 0.578 0.933 0.780 0.010 0.064 Age at FFTP*     <= 21 19.89 (16.98–23.29) 15.53 (13.45–17.94) 8.26 (7.50–9.08) 0.454 (0.386–0.534) 0.248 (0.220–0.280) 451.24 (388.77–523.74) 0.405 (0.339–0.482) 4.15 (3.43–5.03)     22–24 17.46 (15.52–19.63) 14.41 (12.96–16.04) 8.46 (7.88–9.07) 0.466 (0.413–0.526) 0.264 (0.242–0.289) 464.05 (415.71–518.53) 0.484 (0.426–0.552) 4.84 (4.20–5.58)     25–27 17.32 (15.67–19.16) 15.01 (13.69–16.44) 8.06 (7.58–8.57) 463 (0.417–0.513) 0.257 (0.238–0.277) 445.41 (405.05–489.80) 0.484 (0.433–0.541) 4.65 (4.12–5.26)     >= 28 18.67 (16.63–20.99) 15.07 (13.56–16.76) 8.84 (8.24–9.48) 0.466 (0.414–0.525) 0.258 (0.237–0.282) 452.59 (405.45–504.72) 0.459 (0.404–0.522) 4.73 (4.11–5.45)     P 0.741 0.988 0.389 0.858 0.823 0.850 0.483 0.444 Age at menopause∞     <= 47 18.16 (16.20- 20.37) 14.74 (13.30–16.33) 8.38 (7.81–8.98) 0.511 (0.456–0.572) 0.281 (0.258–0.306) 445.41 (401.01–494.72) 0.461(0.402–0.530) 4.94 (4.35–5.61)     48–50 19.26 (17.53–21.16) 15.41 (14.17–16.76) 8.61 (8.12–9.15) 0.484 (0.441–0.532) 0.259 (0.241–0.277) 481.06 (441.42–524.79) 0.447 (0.399–0.501) 4.38 (3.95–4.86)     51–52 16.76 (14.86–8.89) 16.23 (14.57–18.06) 8.36 (7.77–8.99) 0.472 (0.419–0.532) 0.261 (0.238–0.285) 462.20 (413.64–516.46) 0.499 (0.431–0.577) 5.01 (4.38–5.72)     >= 53 16.96 (15.01–19.18) 14.47 (12.96–16.13) 8.50 (7.88–9.15) 0.411 (0.364–0.464) 0.238 (0.217–0.261) 416.55 (372.41–466.38) 0.501 (0.432–0.580) 4.65 (4.06–5.33)     P 0.336 0.770 0.854 0.024 0.015 0.645 0.374 0.690 †  § The always irregular category was not included when testing for trend  * Among parous participants  ∞ Number of years since menopause was not included in the model Discussion The results of the present study suggest that the “lifetime cumulative number of cycles” is related to SHBG and that some reproductive characteristics are also associated to postmenopausal serum sex hormone levels. We noted significant associations after adjusting for important confounders, the “lifetime cumulative number of menstrual cycles” related inversely with levels of SHBG and positively with SHBG/E2. Age at menarche directly relates to both DHEAS and androstenedione. Number of children had a significant relation with levels of SHBG and SHBG/E2 and age at menopause was indirectly associated to androstenedione and testosterone. 6 13 15 24 27 8 28 29 17 18 1 10 30 2 4 31 32 33 34 15 14 16 35 36 7 10 37 38 P 14 P P 28 29 38 39 40 41 42 12 14 16 43 44 12 14 16 15 13 To appreciate the findings of our study, some strengths and limitations need to be mentioned. The main advantage is our large sample size, to the best of our knowledge this is the largest study assessing the relationship between reproductive risk factors and serum sex hormone levels in naturally menopausal women and the first one evaluating the relationship between sex hormones and an index that can be used as a proxy of ovarian exposure. Another advantage is that we attempted to adjust for all relevant covariates, in order to obtain fully adjusted estimates. The participants in this cross-sectional study originated from a random sample of a large cohort, and blood samples where drawn from 97.5% of the 17,357 participants, favoring the generalization of our results. Given the prospective nature of the Prospect-EPIC study, and because participants were recruited through an existing population-based breast cancer screening program, we included only disease-free participants at baseline blood collection. 45 46 In summary, in this group of naturally postmenopausal women, the “lifetime cumulative number menstrual cycles” was related only to SHBG, suggesting that free levels of estrogens or androgens may be related to this index, reflecting lifetime exposure to ovarian hormones. Our findings show that some reproductive characteristics are related to postmenopausal serum sex hormone levels, however we do not find clear evidence to explain the mechanism by which reproductive risk factors influence hormone-related cancers in naturally postmenopausal women. It seems that reproductive and menstrual characteristics do not affect postmenopausal breast cancer risk via a long-term effect on sex hormone levels. The association of reproductive risk factors and breast cancer could be a result of prolonged exposure to ovarian hormones, and not only the product of a direct, long-lasting effect of sex hormone serum concentrations. Further research is needed in order to unravel the biological mechanisms through which reproductive characteristics relate to breast cancer risk.