Table 2.
Associations between reproductive factors and breast cancer in PLCO study.
| Reproductive factors | N , (%) | Breast cancer | Breast cancer | ||||
|---|---|---|---|---|---|---|---|
| % developed breast cancer | Crude HR (95% CI) | P-valuea | Adjusted HRb (95% CI) | P-valuea | |||
| Ever take oral contraceptive pills | |||||||
| No | 32237 (45.7) | 5.7 | Ref | Ref | |||
| Yes | 38374 (54.4) | 5.8 | 1.03 (0.97, 1.10) | 0.304 | 1.07 (0.99, 1.15) | 0.082 | |
| Menopausal hormone therapy use | |||||||
| Never | 23092 (32.9) | 4.9 | Ref | Ref | |||
| Former user | 11493 (16.4) | 5.2 | 1.05 (0.95, 1.16) | 0.351 | 1.06 (0.95, 1.19) | 0.275 | |
| Current user less than 5 years | 11722 (16.7) | 6.2 | 1.24 (1.13, 1.37) | <0.001 | 1.22 (1.10, 1.37) | 0.000 | |
| Current user more than 5 years | 23842 (34.0) | 6.6 | 1.35 (1.25, 1.46) | <0.001 | 1.44 (1.31, 1.58) | <0.001 | |
| Age at birth of first child | |||||||
| ≤19 | 12052 (18.9) | 5.0 | Ref | Ref | |||
| 20-24 | 32817 (51.3) | 5.4 | 1.07 (0.97, 1.17) | 0.182 | 0.86 (0.72, 1.01) | 0.072 | |
| 25-29 | 14165 (22.2) | 6.1 | 1.19 (1.07, 1.32) | 0.001 | 0.88 (0.70, 1.10) | 0.259 | |
| 30-34 | 3650 (5.7) | 7.1 | 1.40 (1.21, 1.62) | <0.001 | 1.05 (0.78, 1.40) | 0.766 | |
| ≥35 | 1239 (1.9) | 7.0 | 1.40 (1.12, 1.75) | 0.003 | 0.84 (0.55, 1.29) | 0.433 | |
| Numbers of early termination of pregnancy | |||||||
| None | 46377 (65.8) | 5.8 | Ref | Ref | |||
| One | 15539 (22.0) | 5.6 | 0.97 (0.90, 1.05) | 0.507 | 1.03 (0.95, 1.13) | 0.485 | |
| Two or more | 8579 (12.2) | 5.8 | 1.01 (0.92, 1.11) | 0.885 | 1.12 (0.99, 1.26) | 0.080 | |
| Ever been pregnant | |||||||
| No | 5218 (7.4) | 7.0 | Ref | Ref | |||
| Yes | 65421 (92.6) | 5.6 | 0.79 (0.71, 0.88) | <0.001 | - | ||
| Age in years when first became pregnant | |||||||
| ≤19 | 16472 (25.2) | 4.9 | Ref | Ref | |||
| 20-24 | 32608 (49.9) | 5.6 | 1.13 (1.04, 1.23) | 0.005 | 1.19 (1.02, 1.38) | 0.031 | |
| 25-29 | 12490 (19.1) | 6.2 | 1.25 (1.13, 1.38) | <0.001 | 1.19 (0.96, 1.48) | 0.114 | |
| 30-34 | 2841 (4.4) | 6.9 | 1.40 (1.20, 1.64) | <0.001 | 1.14 (0.83, 1.56) | 0.415 | |
| ≥35 | 894 (1.4) | 8.2 | 1.68 (1.32, 2.14) | <0.001 | 1.68 (1.03, 2.77) | 0.039 | |
| Number of pregnancies | |||||||
| None | 5218 (7.4) | 7.0 | Ref | Ref | |||
| One | 4036 (5.7) | 6.1 | 0.88 (0.75, 1.04) | 0.130 | - | ||
| Two | 12481 (17.7) | 6.0 | 0.85 (0.75, 0.96) | 0.010 | 0.95 (0.81, 1.12) | 0.539 | |
| Three to four | 28652 (40.6) | 5.7 | 0.79 (0.71, 0.89) | <0.001 | 0.93 (0.79, 1.10) | 0.399 | |
| Five to nine | 18800 (26.6) | 5.3 | 0.73 (0.65, 0.83) | <0.001 | 0.88 (0.74, 1.05) | 0.166 | |
| More than ten | 1391 (2.0) | 4.5 | 0.62 (0.47, 0.81) | 0.001 | 0.73 (0.53, 1.01) | 0.055 | |
| Ever tried to become pregnant for a year or more | |||||||
| No | 60284 (85.6) | 5.6 | Ref | Ref | |||
| Yes | 10140 (14.4) | 6.4 | 1.14 (1.05, 1.24) | 0.002 | 1.08 (0.98, 1.20) | 0.124 | |
| Age at menarche | |||||||
| ≤11 years | 14294 (20.3) | 6.1 | Ref | Ref | |||
| 12-13 years | 37922 (53.8) | 5.7 | 0.92 (0.85, 1.00) | 0.048 | 0.92 (0.84, 1.00) | 0.058 | |
| 14-15 years | 15107 (21.4) | 5.6 | 0.90 (0.82, 0.99) | 0.031 | 0.91 (0.82, 1.02) | 0.093 | |
| ≥16 years | 3219 (4.6) | 5.3 | 0.87 (0.74, 1.03) | 0.096 | 0.86 (0.72, 1.04) | 0.120 | |
| Age at menopause | |||||||
| <40 | 9727 (13.9) | 4.8 | Ref | Ref | |||
| 40-44 | 9880 (14.1) | 5.3 | 1.12 (0.99, 1.27) | 0.079 | 1.08 (0.94, 1.25) | 0.262 | |
| 45-49 | 16733 (23.9) | 5.4 | 1.13 (1.01, 1.26) | 0.034 | 1.06 (0.93, 1.21) | 0.405 | |
| 50-54 | 25831 (36.8) | 6.0 | 1.24 (1.12, 1.38) | <0.001 | 1.15 (1.00, 1.33) | 0.043 | |
| ≥55 | 7955 (11.3) | 7.0 | 1.44 (1.27, 1.63) | <0.001 | 1.24 (1.06, 1.46) | 0.009 | |
| Menopausal type | |||||||
| Natural Menopause | 43703 (63.0) | 5.9 | Ref | Ref | |||
| Radiation/Drug Therapy | 23329 (33.6) | 5.1 | 1.38 (1.19, 1.60) | <0.001 | 1.15 (0.97, 1.36) | 0.119 | |
| Surgery | 2355 (3.4) | 8.0 | 0.88 (0.82, 0.94) | <0.001 | 0.86 (0.78, 0.95) | 0.004 | |
Abbreviations: PLCO, Prostate, Lung, Colorectal, and Ovarian study; HR, hazard ratio; CI, confidence interval. a Significant associations are in bold. b Adjusted for age, enrollment year, study centers, race, educational level, marital status, family female breast cancer history, current BMI status, benign breast diseases history, and all the other reproductive factors.