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Table 2 Age- and multivariable-adjusted risk ratios (RR) and 95 % confidence intervals (95 % CI) for associations of reproductive risk factors and contralateral breast cancer (CBC)

From: Reproductive factors, tumor estrogen receptor status and contralateral breast cancer risk: results from the WECARE study

 

Cases, N (%)

Controls, N (%)

Age-adjusteda RR (95 % CI)

Multivariableb RR (95 % CI)

Age at menarche (years)

 ≤11

305 (20)

437 (20)

1.0 (ref.)

1.0 (ref.)

 12

419 (28)

528 (24)

1.16 (0.93, 1.44)

1.16 (0.93, 1.44)

 13

404 (27)

589 (27)

0.95 (0.77, 1.18)

0.96 (0.77, 1.19)

 ≥14

387 (25)

650 (29)

0.81 (0.65, 1.01)

0.82 (0.65, 1.03)

 P dtrend

  

0.009

0.02

Full-term pregnancies at first diagnosis

 None

311 (20)

399 (18)

1.0 (ref.)

1.0 (ref.)

 ≥1

1208 (79)

1810 (82)

0.91 (0.76, 1.08)

0.92 (0.77, 1.11)

Full-term pregnancies between first diagnosis and reference date

 Parous at first diagnosis, no interval pregnancy

1189 (78)

1785 (81)

1.0 (ref.)

1.0 (ref.)

 Nulliparous at reference date

311 (20)

399 (18)

1.11 (0.93, 1.33)

0.96 (0.76, 1.21)

 Full-term pregnancy between first diagnosis and reference date

19 (1)

25 (1)

1.30 (0.63, 2.66)

0.96 (0.76, 1.21)

Number of full-term pregnancies at first diagnosisc

 1

264 (22)

341 (19)

1.0 (ref.)

1.0 (ref.)

 2

572 (47)

847 (47)

0.93 (0.74, 1.19)

0.97 (0.75, 1.25)

 3

260 (22)

390 (22)

0.80 (0.61, 1.05)

0.87 (0.65, 1.17)

 ≥4

112 (9)

232 (13)

0.57 (0.41, 0.81)

0.60 (0.41, 0.88)

 P dtrend

  

0.001

0.005

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

 <20

175 (14)

276 (15)

1.0 (ref.)

1.0 (ref.)

 20–24

444 (37)

667 (37)

1.26 (0.97, 1.65)

1.19 (0.90, 1.58)

 25–29

355 (29)

550 (30)

1.10 (0.83, 1.45)

1.01 (0.75, 1.35)

 ≥30

234 (19)

317 (18)

1.36 (0.99, 1.86)

1.20 (0.84, 1.70)

 P dtrend

  

0.25

0.82

Breastfeeding at reference date (months)c

 Never

680 (45)

940 (43)

1.0 (ref.)

1.0 (ref.)

 Ever

837 (55)

1268 (57)

0.86 (0.75, 1.00)

0.84 (0.69, 1.02)

 ≤6

430 (28)

643 (29)

0.91 (0.77, 1.08)

0.88 (0.71, 1.09)

 7–12

185 (12)

270 (12)

0.94 (0.74, 1.19)

0.91 (0.69, 1.19)

 13–24

141 (9)

232 (10)

0.74 (0.58, 0.95)

0.72 (0.54, 0.96)

 ≥25

81 (5)

123 (6)

0.78 (0.56, 1.07)

0.77 (0.55, 1.10)

 P dtrend

  

0.02

0.06

Time since last full-term pregnancy, at reference date (years)c

 <5

37 (3)

50 (3)

1.12 (0.66, 1.90)

1.19 (0.66, 2.14)

 5–<10

57 (5)

122 (7)

0.61 (0.41, 0.93)

0.63 (0.40, 1.01)

 10 –<15

129 (11)

195 (11)

0.89 (0.65, 1.20)

0.93 (0.66, 1.31)

 15–<20

203 (17)

305 (17)

0.96 (0.76, 1.21)

0.99 (0.76, 1.29)

 ≥20

782 (65)

1138 (63)

1.0 (ref.)

1.0 (ref.)

 P dtrend

  

0.26

0.49

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

 <10

407 (34)

649 (36)

1.0 (ref.)

1.0 (ref.)

 10–14

422 (35)

637 (35)

1.16 (0.94, 1.43)

1.20 (0.96, 1.49)

 15–19

239 (20)

346 (19)

1.12 (0.87, 1.45)

1.10 (0.83, 1.44)

 ≥20

136 (11)

170 (9)

1.35 (0.97, 1.87)

1.33 (0.94, 1.88)

 P dtrend

  

0.07

0.12

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

 Premenopausal

1124 (74)

1676 (76)

1.0 (ref.)

1.0 (ref.)

 Postmenopausal/<45 years

195 (13)

282 (13)

1.10 (0.88, 1.38)

1.03 (0.81, 1.32)

 Postmenopausal/≥45 years

194 (13)

240 (11)

1.27 (0.98, 1.64)

1.19 (0.91, 1.57)

  1. aIncluding offset for countermatching
  2. bAdjusted for age at first diagnosis, histology at first diagnosis, family history, stage at first diagnosis, chemotherapy/hormonal treatment for first diagnosis, use of postmenopausal hormone therapy up to first diagnosis, age at menarche, age at menopause and parity, with offset for countermatching
  3. cAmong parous women only, additionally mutually adjusted for age at first FTP, number of FTP, breast-feeding
  4. d P trend calculated by modeling category medians continuously
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