Table 2.
Multivariable-adjusteda hazard ratios (HRs) and 95% confidence intervals for breast cancer overall and by ER status in relation to parity and breastfeeding: Nurses’ Health Study (1976–2012) and Nurses’ Health Study II (1989–2013)
| All | ER+ | ER− | Phet by | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Cases | PY | HR (95% CI) | Cases | PY | HR (95% CI) | Cases | PY | HR (95% CI) | subtype | |
| Parous | ||||||||||
| Nulliparous | 1498 | 742,502 | 1 (ref.) | 979 | 742,976 | 1 (ref.) | 209 | 743,672 | 1 (ref.) | |
| Parous | 10,954 | 4,540,107 | 0.84 (0.80–0.89) | 7256 | 4,543,555 | 0.82 (0.77–0.88) | 1769 | 4,548,669 | 0.98 (0.84–1.13) | 0.03 |
| Breastfeeding | ||||||||||
| Never breastfed (parous + nulliparous) | 5101 | 2,100,966 | 1 (ref.) | 3271 | 2,102,676 | 1 (ref.) | 835 | 2,104,928 | 1 (ref.) | |
| Ever breastfed | 7351 | 3,181,643 | 0.94 (0.91–0.98) | 4964 | 3,183,855 | 0.99 (0.94–1.04) | 1143 | 3,187,412 | 0.83 (0.75–0.92) | 0.002 |
| ≤ 6 months | 3242 | 1,255,908 | 0.93 (0.88–0.97) | 2177 | 1,256,886 | 0.96 (0.90–1.01) | 517 | 1,258,482 | 0.84 (0.75–0.95) | |
| 7–11 months | 1203 | 535,074 | 0.96 (0.90–1.03) | 819 | 535,428 | 1.03 (0.95–1.11) | 180 | 535,998 | 0.81 (0.68–0.95) | |
| ≥ 12 months | 2906 | 1,390,661 | 0.96 (0.91–1.01) | 1968 | 1,391,541 | 1.02 (0.96–1.09) | 446 | 1,392,932 | 0.83 (0.73–0.94) | |
| Ptrend = 0.72 | Ptrend = 0.22 | Ptrend = 0.05 | 0.02 | |||||||
| Parity/breastfeeding | ||||||||||
| Nulliparous | 1498 | 742,502 | 1 (ref.) | 979 | 742,976 | 1 (ref.) | 209 | 743,672 | 1 (ref.) | |
| Parous women, never breastfed | 3603 | 1,358,464 | 0.88 (0.83–0.94) | 2292 | 1,359,700 | 0.83 (0.77–0.90) | 626 | 1,361,257 | 1.11 (0.94–1.31) | < 0.001b |
| 1 child | 518 | 210,088 | 0.92 (0.83–1.02) | 332 | 210,263 | 0.88 (0.78–1.00) | 91 | 210,483 | 1.15 (0.90–1.48) | 0.002c |
| 2 children | 1386 | 516,819 | 0.93 (0.86–1.00) | 870 | 517,287 | 0.86 (0.78–0.94) | 250 | 517,868 | 1.20 (0.99–1.45) | |
| ≥ 3 children | 1699 | 631,558 | 0.81 (0.76–0.87) | 1090 | 632,150 | 0.77 (0.70–0.84) | 285 | 632,906 | 1.03 (0.85–1.25) | |
| Parous women, ever breastfed | 7351 | 3,181,643 | 0.83 (0.78–0.88) | 4964 | 3,183,855 | 0.82 (0.76–0.88) | 1143 | 3,187,412 | 0.92 (0.79–1.08) | |
| 1 child | 666 | 313,938 | 0.95 (0.86–1.04) | 437 | 314,149 | 0.93 (0.83–1.04) | 97 | 314,457 | 0.97 (0.76–1.23) | |
| 2 children | 2493 | 1,166,539 | 0.85 (0.79–0.91) | 1694 | 1,167,283 | 0.85 (0.78–0.92) | 362 | 1,168,492 | 0.87 (0.73–1.03) | |
| ≥3 children | 4192 | 1,701,166 | 0.79 (0.74–0.84) | 2833 | 1,702,423 | 0.78 (0.72–0.84) | 684 | 1,704,463 | 0.95 (0.81–1.12) | |
| Among parous women | ||||||||||
| Parityd | ||||||||||
| 1 child | 1184 | 524,026 | 1 (ref.) | 769 | 524,412 | 1 (ref.) | 188 | 524,940 | 1 (ref.) | |
| 2 children | 3879 | 1,683,357 | 1.02 (0.95–1.09) | 2564 | 1,684,570 | 1.02 (0.94–1.11) | 612 | 1,686,360 | 1.02 (0.86–1.21) | |
| 3 children | 3165 | 1,259,779 | 1.03 (0.96–1.10) | 2120 | 1,260,791 | 1.03 (0.94–1.13) | 502 | 1,262,311 | 1.06 (0.88–1.27) | |
| ≥ 4 children | 2726 | 1,072,945 | 0.92 (0.85–1.00) | 1803 | 1,073,782 | 0.91 (0.82–1.00) | 467 | 1,075,059 | 1.08 (0.89–1.31) | |
| Ptrend = 0.0001 | Ptrend = 0.0001 | Ptrend = 0.43 | 0.02 | |||||||
| Breastfeedingd | ||||||||||
| Never breastfed | 3603 | 1,358,464 | 1 (ref.) | 2292 | 1,359,700 | 1 (ref.) | 626 | 1,361,257 | 1 (ref.) | |
| Ever breastfed | 7351 | 3,181,643 | 0.95 (0.91–0.99) | 4964 | 3,183,855 | 0.99 (0.94–1.05) | 1143 | 3,187,412 | 0.82 (0.74–0.91) | < 0.001 |
| ≤ 6 months | 3242 | 1,255,908 | 0.93 (0.89–0.98) | 2177 | 1,256,886 | 0.96 (0.91–1.02) | 517 | 1,258,482 | 0.84 (0.75–0.95) | |
| 7–11 months | 1203 | 535,074 | 0.95 (0.89–1.02) | 819 | 535,428 | 1.02 (0.94–1.10) | 180 | 535,998 | 0.79 (0.67–0.94) | |
| ≥ 12 months | 2906 | 1,390,661 | 0.97 (0.92–1.02) | 1968 | 1,391,541 | 1.04 (0.97–1.11) | 446 | 1,392,932 | 0.80 (0.70–0.92) | |
| Ptrend, incl never = 0.70 | Ptrend, incl never = 0.05(+) | Ptrend, incl never = 0.02 | 0.002 | |||||||
| Ptrend, among ever = 0.10 | Ptrend, among ever = 0.02(+) | Ptrend, among ever = 0.35 | ||||||||
aAdjusted for age, height (< 1.60, 1.60 to < 1.65, 1.65 to < 1.70, 1.70 to < 1.75, 1.75+ meters), BMI at age 18 (< 20, 20–21.9, 22–23.9, 24–26.9, 27+ kg/m2), weight change since age 18 (continuous, kg), age at menarche (< 12, 12, 13, 14, 15+ years), menopausal status (premenopausal, postmenopausal or unknown), age at natural menopause (continuous), HT use (never, past, current E only, current E + P, current other), alcohol consumption (non-drinker, < 5, 5–10, 10–15, 15+ g/day), total physical activity (< 3, 3–9, 9–18, 18–27, 27+ MET-h/day), family history of breast cancer (yes/no), history of benign breast disease (yes/no)
bPhet for nulliparous and parous ever vs. never breastfed
cPhet for nulliparous and number of children by ever vs. never breastfed
dAmong parous women only, additionally adjusted for age at first birth (continuous, years), and parity and breastfeeding mutually adjusted