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Table 3.

Parity and breast cancer risk stratified by hormone receptor expression

First author (reference)Country (study)Cases (n)Births (n)*Risk estimate (95% CI)
    ER+ ER− 
Wohlfahrt (22) Denmark (DBCG) 6,044 1+ 0.8 (0.7-0.8) 0.9 (0.8-1.0) 
Hildreth (42) United States 148 1+ 0.6 (0.3-1.1) 2.5 (0.8-10.0) 
Cooper (36) Australia 380 1.4 (0.7-2.7) 1.3 (0.5-3.4) 
   0.8 (0.5-1.4) 0.8 (0.4-1.9) 
   3+ 0.8 (0.5-1.4) 1.0 (0.4-2.3) 
Hislop (37) Canada (BC) 512 1-2 0.8 (0.6-1.0) 0.9 (0.6-1.4) 
   3+ 0.7 (0.6-1.0) 1.3 (0.9-2.0) 
McTiernan (39) United States (CASH-WA) 240 1-2 0.8 1.9 
   3+ 0.6 (0.3-1.5) 1.4 (0.6-3.6) 
Stanford (38) United States (CASH-GA) 458 1-2 1.0 (0.6-1.6) 0.9 (0.6-1.5) 
   3-4 0.7 (0.4-1.2) 0.8 (0.5-1.3) 
   5+ 0.5 (0.2-1.0) 1.0 (0.5-1.9) 
Kreiger (35) Canada (ON) 528 1-3 0.7 (0.5-0.9) 0.9 (0.5-1.6) 
   4+ 0.7 (0.4-1.1) 1.4 (0.6-3.0) 
Yoo (41) Japan 455 Per 1 child 1.0 (0.8-1.1) 0.9 (0.8-1.2) 
    PR+ PR− 
Kreiger (35) Canada (ON) 528 1-3 0.6 (0.4-0.9) 0.9 (0.6-1.5) 
   4+ 0.5 (0.3-0.8) 1.6 (0.9-3.1) 
Yoo (41) Japan 455 Per 1 child 0.9 (0.7-1.1) 1.0 (0.9-1.2) 
    ER+/PR+ ER−/PR− 
McCredie (23) Melbourne (Australia) 618 1.1 (0.7-1.7) 1.3 (0.8-2.3) 
   1.0 (0.7-1.5) 1.0 (0.6-1.7) 
   3+ 1.0 (0.5-1.2) 0.8 (0.4-1.3) 
Cotterchio (24) Canada (ON) 3,276 Premenopause   
   0.6 (0.4-1.1) 1.0 (0.5-2.0) 
   0.7 (0.4-1.0) 1.1 (0.6-2.0) 
   3+ 0.4 (0.3-0.8) 0.9 (0.5-1.8) 
   Postmenopause   
   1.0 (0.7-1.4) 0.9 (0.6-1.6) 
   0.8 (0.6-1.2) 0.7 (0.5-1.2) 
   3+ 0.7 (0.5-1.0) 0.7 (0.5-1.1) 
Britton (29) United States (WISH) 1,212 1+ 0.8 (0.6-1.1) 0.8 (0.5-1.2) 
Potter (17) United States (IWHS) 610 1-2 0.8 (0.5-1.1) 2.1 (0.6-6.9) 
   3+ 0.8 (0.5-1.1) 2.2 (0.7-7.2) 
Yoo (41) Japan 455 Per 1 child 1.0 (0.8-1.2) 1.0 (0.8-1.2) 
First author (reference)Country (study)Cases (n)Births (n)*Risk estimate (95% CI)
    ER+ ER− 
Wohlfahrt (22) Denmark (DBCG) 6,044 1+ 0.8 (0.7-0.8) 0.9 (0.8-1.0) 
Hildreth (42) United States 148 1+ 0.6 (0.3-1.1) 2.5 (0.8-10.0) 
Cooper (36) Australia 380 1.4 (0.7-2.7) 1.3 (0.5-3.4) 
   0.8 (0.5-1.4) 0.8 (0.4-1.9) 
   3+ 0.8 (0.5-1.4) 1.0 (0.4-2.3) 
Hislop (37) Canada (BC) 512 1-2 0.8 (0.6-1.0) 0.9 (0.6-1.4) 
   3+ 0.7 (0.6-1.0) 1.3 (0.9-2.0) 
McTiernan (39) United States (CASH-WA) 240 1-2 0.8 1.9 
   3+ 0.6 (0.3-1.5) 1.4 (0.6-3.6) 
Stanford (38) United States (CASH-GA) 458 1-2 1.0 (0.6-1.6) 0.9 (0.6-1.5) 
   3-4 0.7 (0.4-1.2) 0.8 (0.5-1.3) 
   5+ 0.5 (0.2-1.0) 1.0 (0.5-1.9) 
Kreiger (35) Canada (ON) 528 1-3 0.7 (0.5-0.9) 0.9 (0.5-1.6) 
   4+ 0.7 (0.4-1.1) 1.4 (0.6-3.0) 
Yoo (41) Japan 455 Per 1 child 1.0 (0.8-1.1) 0.9 (0.8-1.2) 
    PR+ PR− 
Kreiger (35) Canada (ON) 528 1-3 0.6 (0.4-0.9) 0.9 (0.6-1.5) 
   4+ 0.5 (0.3-0.8) 1.6 (0.9-3.1) 
Yoo (41) Japan 455 Per 1 child 0.9 (0.7-1.1) 1.0 (0.9-1.2) 
    ER+/PR+ ER−/PR− 
McCredie (23) Melbourne (Australia) 618 1.1 (0.7-1.7) 1.3 (0.8-2.3) 
   1.0 (0.7-1.5) 1.0 (0.6-1.7) 
   3+ 1.0 (0.5-1.2) 0.8 (0.4-1.3) 
Cotterchio (24) Canada (ON) 3,276 Premenopause   
   0.6 (0.4-1.1) 1.0 (0.5-2.0) 
   0.7 (0.4-1.0) 1.1 (0.6-2.0) 
   3+ 0.4 (0.3-0.8) 0.9 (0.5-1.8) 
   Postmenopause   
   1.0 (0.7-1.4) 0.9 (0.6-1.6) 
   0.8 (0.6-1.2) 0.7 (0.5-1.2) 
   3+ 0.7 (0.5-1.0) 0.7 (0.5-1.1) 
Britton (29) United States (WISH) 1,212 1+ 0.8 (0.6-1.1) 0.8 (0.5-1.2) 
Potter (17) United States (IWHS) 610 1-2 0.8 (0.5-1.1) 2.1 (0.6-6.9) 
   3+ 0.8 (0.5-1.1) 2.2 (0.7-7.2) 
Yoo (41) Japan 455 Per 1 child 1.0 (0.8-1.2) 1.0 (0.8-1.2) 

NOTE: Adjustment for other reproductive variables was sometimes unclear. Hildreth et al., McTiernan et al., and Yoo et al. adjusted for age at first birth and breast-feeding, assigning nulliparous women a value of zero in the model.

*

Reference group is nulliparous women.

ORs were the reported risk estimates for case-control studies and RRs for cohort designs.

Age-adjusted only.

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