Table 3.
Multivariate-adjusted ORs and 95% confidence intervals (CIs) for associations between fertility factors and risk of invasive breast cancer by tumor subtype in the younger group (≤40 years).
| Reproductive characteristics | Controls | Luminal A | Luminal B | HER2 enriched | TNBC | ||||
|---|---|---|---|---|---|---|---|---|---|
| n (%) | N (%) | OR (95% CI) | N (%) | OR (95% CI) | N (%) | OR (95% CI) | n (%) | OR (95% CI) | |
| Age at menarche | |||||||||
| ≤13 | 466 (47.4) | 42 (48.8) | 1.00 (reference) | 204 (48.6) | 1.00 (reference) | 38 (50.7) | 1.00 (reference) | 68 (50.0) | 1.00 (reference) |
| 14 | 236 (24.0) | 21 (24.4) | 0.99 (0.57-1.71) | 92 (21.9) | 0.93 (0.69-1.26) | 17 (22.7) | 0.93 (0.51-1.69) | 25 (18.4) | 0.74 (0.46-1.21) |
| ≥15 | 281 (28.6) | 23 (26.7) | 0.91 (0.54-1.56) | 124 (29.5) | 1.03 (0.79-1.35) | 20 (26.7) | 0.93 (0.53-1.64) | 43 (31.6) | 1.08 (0.72-1.64) |
| P trend | 0.750 | 0.883 | 0.782 | 0.836 | |||||
| P value for homogeneity of trends | 0.766 | 0.888 | 0.755 | ||||||
| Parity vs. no parity | |||||||||
| Never | 52 (5.3) | 10 (11.6) | 1.00 (reference) | 47 (11.2) | 1.00 (reference) | 5 (6.7) | 1.00 (reference) | 10 (7.4) | 1.00 (reference) |
| Ever | 931 (94.7) | 76 (88.4) | 0.40 (0.18-0.88) | 373 (88.8) | 0.40 (0.25-0.62) | 70 (93.3) | 0.76 (0.27-2.11) | 126 (92.6) | 0.85 (0.40-1.81) |
| 1 | 599 (60.9) | 54 (62.8) | 0.44 (0.20-0.98) | 237 (56.4) | 0.41 (0.26-0.65) | 37 (49.3) | 0.68 (0.24-1.92) | 88 (64.7) | 0.93 (0.43-1.99) |
| ≥2 | 332 (33.8) | 22 (25.6) | 0.32 (0.14-0.77) | 136 (32.4) | 0.38 (0.23-0.61) | 33 (44.0) | 0.92 (0.32-2.65) | 38 (27.9) | 0.69 (0.31-1.55) |
| P trend | 0.029 | 0.005 | 0.484 | 0.178 | |||||
| P value for homogeneity of trends | 0.316 | 0.015 | 0.431 | ||||||
| Number of live births | |||||||||
| 1 | 599 (64.3) | 54 (71.1) | 1.00 (reference) | 237 (63.5) | 1.00 (reference) | 37 (52.9) | 1.00 (reference) | 88 (69.8) | 1.00 (reference) |
| ≥2 | 332 (35.7) | 22 (28.9) | 0.73 (0.44-1.23) | 136 (36.5) | 0.93 (0.72-1.20) | 33 (47.1) | 1.40 (0.85-2.30) | 38 (30.2) | 0.74 (0.49-1.11) |
| Age at first live birth | |||||||||
| <25 | 348 (37.4) | 37 (48.7) | 1.00 (reference) | 157 (42.1) | 1.00 (reference) | 28 (40.0) | 1.00 (reference) | 52 (41.3) | 1.00 (reference) |
| 25-29 | 488 (52.4) | 34 (44.7) | 0.65 (0.40-1.05) | 183 (49.1) | 0.88 (0.68-1.14) | 35 (50.0) | 0.99 (0.59-1.67) | 63 (50.0) | 0.92 (0.62-1.36) |
| ≥30 | 95 (10.2) | 5 (6.6) | 0.48 (0.18-1.27) | 33 (8.8) | 0.78 (0.50-1.22) | 7 (10.0) | 1.05 (0.44-2.54) | 11 (8.7) | 0.86 (0.43-1.73) |
| P trend | 0.043 | 0.209 | 0.949 | 0.598 | |||||
| P value for homogeneity of trends | 0.263 | 0.243 | 0.293 | ||||||
| Breastfeeding (parous women) | |||||||||
| Never | 66 (7.1) | 7 (9.2) | 1.00 (reference) | 41 (11.0) | 1.00 (reference) | 6 (8.6) | 1.00 (reference) | 14 (11.1) | 1.00 (reference) |
| Ever | 865 (92.9) | 69 (90.8) | 0.75 (0.33-1.70) | 332 (89.0) | 0.58 (0.38-0.88) | 64 (91.4) | 0.80 (0.33-1.94) | 112 (88.9) | 0.62 (0.34-1.15) |
| <6 months | 59 (6.3) | 5 (6.6) | 0.75 (0.22-2.50) | 30 (8.0) | 0.69 (0.38-1.25) | 5 (7.1) | 0.83 (0.24-2.91) | 15 (11.9) | 1.11 (0.49-2.52) |
| 6–11 months | 141 (15.1) | 13 (17.1) | 0.82 (0.31-2.17) | 59 (15.8) | 0.67 (0.40-1.10) | 12 (17.1) | 1.06 (0.38-3.00) | 25 (19.8) | 0.84 (0.41-1.72) |
| ≥12 months | 665 (71.4) | 51 (67.1) | 0.73 (0.32-1.68) | 243 (65.1) | 0.55 (0.36-0.84) | 47 (67.1) | 0.75 (0.30-1.83) | 72 (57.1) | 0.52 (0.28-0.99) |
| P trend | 0.492 | 0.005 | 0.428 | 0.004 | |||||
| P value for homogeneity of trends | 0.580 | 0.962 | 0.209 | ||||||
All odds ratios (OR) are adjusted for 5-year age group, year of recruitment (2012, 2013, 2014, 2015, 2016, and 2017), oral contraceptive use (yes, no), BMI (<23, 23–24.99, 25-27.49, and ≥27.5 kg/m2), and family history of breast cancer (yes, no). Breastfeeding history was adjusted for the number of live births.