Table 3 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

Age at menarche (years)

 ≤11

160 (20)

269 (21)

1.0 (ref.)

99 (21)

106 (19)

1.0 (ref.)

 12

226 (28)

301 (24)

1.25 (0.92, 1.70)

131 (28)

134 (24)

1.07 (0.70, 1.65)

 13

210 (26)

325 (26)

0.94 (0.69, 1.28)

121 (26)

151 (27)

1.04 (0.68, 1.59)

 ≥14

199 (25)

358 (29)

0.93 (0.68, 1.28)

113 (24)

165 (29)

0.70 (0.45, 1.07)

 P ^b_trend

0.27

0.09

0.40

Full-term pregnancies at first diagnosis

 None

173 (22)

248 (20)

1.0 (ref.)

95 (20)

98 (17)

1.0 (ref.)

 ≥1

623 (78)

1004 (80)

0.93 (0.73, 1.19)

371 (79)

462 (82)

0.95 (0.65, 1.39)

0.93

Full-term pregnancies between first diagnosis and reference date

 Parous at first diagnosis, no interval pregnancy

620 (78)

990 (79)

1.0 (ref.)

361 (77)

455 (81)

1.0 (ref.)

 Nulliparous at reference date

173 (22)

248 (20)

0.97 (0.72, 1.30)

95 (20)

98 (17)

1.00 (0.66, 1.51)

 Full-term pregnancy between first diagnosis and reference date

3 (0)

14 (1)

0.21 (0.04, 1.16)

10 (2)

7 (1)

1.88 (0.57, 6.18)

0.07

Number of full-term pregnancies at first diagnosis^c

 1

144 (23)

198 (20)

1.0 (ref.)

72 (19)

100 (22)

1.0 (ref.)

 2

289 (46)

467 (47)

1.05 (0.73, 1.51)

182 (49)

213 (46)

0.97 (0.60, 1.57)

 3

134 (22)

211 (21)

0.97 (0.64, 1.47)

82 (22)

91 (20)

0.98 (0.56, 1.71)

 ≥4

56 (9)

128 (13)

0.52 (0.30, 0.90)

35 (9)

58 (13)

0.60 (0.30, 1.19)

 P ^b_trend

0.02

0.14

0.95

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

 <20

78 (13)

153 (15)

1.0 (ref.)

70 (19)

68 (15)

1.0 (ref.)

 20–24

211 (34)

359 (36)

1.14 (0.74, 1.76)

147 (40)

170 (37)

1.00 (0.59, 1.68)

 25–29

197 (32)

315 (31)

1.21 (0.77, 1.89)

91 (25)

132 (29)

0.56 (0.32, 0.99)

 ≥30

137 (22)

177 (18)

1.43 (0.87, 2.36)

63 (17)

92 (20)

0.66 (0.35, 1.27)

 P ^b_trend

0.18

0.03

0.03

Breastfeeding at reference date (months)^c

 Never

180 (29)

308 (31)

1.0 (ref.)

136 (37)

150 (32)

1.0 (ref.)

 Ever

443 (71)

696 (69)

1.09 (0.81, 1.47)

233 (63)

311 (67)

0.69 (0.48, 1.00)

0.05

 ≤6

224 (36)

350 (35)

1.06 (0.76, 1.48)

111 (30)

172 (37)

0.65 (0.42, 1.00)

 7–12

82 (13)

148 (15)

1.03 (0.66, 1.61)

66 (18)

53 (11)

0.95 (0.55, 1.66)

 13–24

86 (14)

132 (13)

1.09 (0.69, 1.72)

35 (9)

50 (11)

0.83 (0.43, 1.62)

 ≥25

51 (8)

66 (7)

1.53 (0.87, 2.72)

21 (6)

36 (8)

0.48 (0.22, 1.02)

 P ^b_trend

0.19

0.15

0.07

Time since last full-term pregnancy, at reference date (years)^{c, d}

 <15

104 (17)

250 (23)

0.87 (0.58, 1.32)

88 (24)

122 (26)

0.95 (0.59, 1.53)

 15–<20

104 (17)

165 (15)

1.06 (0.74, 1.54)

60 (16)

70 (15)

1.04 (0.65, 1.68)

 ≥20

415 (67)

654 (61)

1.0 (ref.)

223 (60)

270 (58)

1.0 (ref.)

 P ^b_trend

0.65

0.74

0.99

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

 <10

191 (31)

358 (36)

1.0 (ref.)

141 (38)

156 (34)

1.0 (ref.)

 10–14

217 (35)

352 (35)

1.29 (0.93, 1.80)

128 (35)

164 (36)

0.88 (0.58, 1.34)

 15–19

131 (21)

192 (19)

1.38 (0.94, 2.05)

65 (18)

87 (19)

0.86 (0.51, 1.45)

 ≥20

83 (13)

101 (10)

1.47 (0.93, 2.33)

35 (9)

50 (11)

0.58 (0.29, 1.17)

 P ^b_trend

0.06

0.15

0.10

Menopausal status/age at menopause at 2 years before first diagnosis

 Premenopausal

583 (73)

932 (74)

1.0 (ref.)

348 (75)

345 (78)

1.0 (ref.)

 Postmenopausal/<45 years

99 (12)

164 (13)

1.00 (0.72, 1.40)

65 (14)

65 (12)

1.44 (0.92, 2.26)

 Postmenopausal/≥45 years

113 (14)

149 (12)

1.39 (0.98, 1.97)

50 (11)

59 (11)

1.03 (0.62, 1.71)

0.19