Table 4 Multivariable-adjusted risk ratios (RR) and 95 % confidence intervals (95 % CI) for associations of reproductive risk factors and contralateral breast cancer (CBC), by estrogen receptor (ER) status of first breast cancer and menopausal status 2 years prior to first breast cancer diagnosis

Age at menarche (years)

 ≤11

94/205

1.0 (ref.)

65/63

1.0 (ref.)

72/83

1.0 (ref.)

27/23

1.0 (ref.)

 12

166/219

1.74 (1.20, 2.53)

60/81

0.63 (0.36, 1.13)

95/108

0.96 (0.58, 1.58)

35/25

1.75 (0.71, 4.33)

 13

168/236

1.56 (1.08, 2.27)

41/85

0.29 (0.16, 0.52)

97/121

1.12 (0.69, 1.81)

23/30

0.75 (0.30, 1.86)

 ≥14

153/271

1.33 (0.91, 1.93)

46/85

0.45 (0.25, 0.81)

81/120

0.79 (0.48, 1.32)

30/44

0.45 (0.20, 1.04)

 P ^b_trend

0.33

0.001

0.62

0.01

0.0001

Full-term pregnancies at first diagnosis

 None

133/187

1.0 (ref.)

40/60

1.0 (ref.)

76/82

1.0 (ref.)

18/16

1.0 (ref.)

 ≥1

449/744

0.87 (0.65, 1.15)

172/253

1.22 (0.74, 2.00)

272/353

0.97 (0.63, 1.48)

96/107

0.84 (0.36, 1.95)

0.71

Number of full-term pregnancies at first diagnosis^c

 1

111/146

1.0 (ref.)

32/52

1.0 (ref.)

56/78

1.0 (ref.)

15/21

1.0 (ref.)

 2

221/369

0.99 (0.65, 1.49)

67/93

1.53 (0.74, 3.14)

140/175

1.02 (0.60, 1.74)

41/37

1.41 (0.41, 4.93)

 3

89/156

1.00 (0.62, 1.61)

45/53

1.22 (0.54, 2.75)

56/63

1.03 (0.53, 1.98)

25/28

1.57 (0.56, 4.42)

 ≥4

28/73

0.41 (0.20, 0.82)

28/55

0.81 (0.34, 1.94)

20/37

0.64 (0.28, 1.44)

15/21

1.43 (0.44, 4.68)

 P ^b_trend

0.03

0.28

0.28

0.52

0.98

Age at first full-term pregnancy at reference date (years)^c

 <20

46/92

1.0 (ref.)

32/60

1.0 (ref.)

41/44

1.0 (ref.)

29/24

1.0 (ref.)

 20–24

136/252

1.12 (0.63, 1.96)

75/105

1.25 (0.64, 2.47)

104/117

1.14 (0.58, 2.21)

42/53

0.76 (0.32, 1.78)

 25–29

152/246

1.24 (0.70, 2.18)

44/68

1.38 (0.65, 2.95)

72/108

0.61 (0.31, 1.22)

18/23

0.51 (0.18, 1.47)

 ≥30

115/154

1.35 (0.74, 2.47)

21/20

2.30 (0.84, 6.25)

55/84

0.64 (0.30, 1.41)

7/7

0.87 (0.19, 3.92)

 P ^b_trend

0.28

0.13

0.03

0.40

0.17

Breastfeeding at reference date (months)^c

 Never

111/198

1.0 (ref.)

69/106

1.0 (ref.)

98/106

1.0 (ref.)

38/43

1.0 (ref.)

 Ever

338/546

1.26 (0.87, 1.81)

103/147

0.90 (0.52, 1.54)

172/246

0.62 (0.40, 0.96)

58/64

0.72 (0.34, 1.51)

0.07

 ≤6

164/259

1.31 (0.87, 1.97)

58/89

0.74 (0.40, 1.36)

75/127

0.59 (0.35, 1.00)

34/44

0.59 (0.25, 1.38)

 7–12

63/118

1.10 (0.65, 1.87)

19/29

1.13 (0.48, 2.63)

51/43

0.94 (0.49, 1.79)

14/10

0.83 (0.26, 2.67)

 13–24

72/112

1.24 (0.74, 2.08)

14/20

1.33 (0.41, 4.35)

28/46

0.71 (0.35, 1.48)

7/4

1.59 (0.23, 10.8)

 ≥25

39/57

1.78 (0.93, 3.38)

12/9

1.50 (0.37, 6.04)

18/30

0.37 (0.16, 0.86)

3/6

1.23 (0.17, 9.17)

 P ^b_trend

0.20

0.28

0.06

0.71

0.19

Time between menarche and first full-term pregnancy at reference date (years)^c

 <10

119/237

1.0 (ref.)

72/118

1.0 (ref.)

90/102

1.0 (ref.)

50/54

1.0 (ref.)

 10–14

161/260

1.26 (0.85, 1.870

56/91

1.52 (0.82, 2.81)

96/121

0.86 (0.51, 1.44)

31/42

0.91 (0.41, 1.99)

 15–19

103/160

1.32 (0.84, 2.06)

27/31

1.85 (0.84, 4.04)

56/81

0.72 (0.40, 1.29)

9/6

1.74 (0.39, 7.81)

 ≥20

65/86

1.25 (0.74, 2.08)

17/13

3.70 (1.27, 10.9)

28/46

0.46 (0.21, 1.01)

6/3

1.42 (0.26, 7.93)

 P ^b_trend

0.30

0.009

0.05

0.59

0.17