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Table 2 Adjusted RRs for the association between pregnancy-related factors and breast CIS and invasive breast cancer

From: Pregnancy-related factors and the risk of breast carcinoma in situand invasive breast cancer among postmenopausal women in the California Teachers Study cohort

 

Breast CIS

Invasive breast cancer

 

Observed Person-years

Cases No.

Adjusted RR (95% CI)

Observed Person-years

Cases No.

Adjusted RR (95% CI)

All women

      

Ever pregnant

      

   No

90,296

116

Reference

92,927

493

Reference

   Yes

444,276

508

0.85 (0.69 to 1.04)

457,417

2335

0.91 (0.83 to 1.00)

   P for homogeneitya

  

0.55

   

Number of full-term pregnanciesb

      

   1

68,527

91

1.01 (0.77 to 1.33)

70,615

355

0.95 (0.83 to 1.09)

   2

165,472

188

0.84 (0.67 to 1.06)

170,385

878

0.94 (0.84 to 1.04)

   3

112,458

131

0.84 (0.65 to 1.08)

115,629

591

0.88 (0.78 to 0.99)

   ≥ 4

74,458

70

0.69 (0.51 to 0.93)

76,634

373

0.82 (0.72 to 0.94)

   P trend

  

0.008

  

0.003

   P for homogeneitya

  

0.26

   

   Ever pregnant, incomplete pregnancies only or outcome of pregnancies unknown

23,361

28

 

24,154

138

 

Parous women only c, d

      

Number of full-term pregnancies

      

   1

68,338

90

Reference

70,426

355

Reference

   2

164,995

188

0.92 (0.70 to 1.20)

169,889

875

1.00 (0.88 to 1.14)

   3

112,082

131

0.93 (0.69 to 1.26)

115,253

590

0.95 (0.82 to 1.10)

   ≥ 4

74,291

70

0.77 (0.54 to 1.11)

76,467

373

0.90 (0.77 to 1.07)

   P trend

  

0.22

  

0.15

   P for homogeneitya

  

0.61

   

Age at first full-term pregnancies, years

      

   < 21

46,965

44

Reference

48,165

203

Reference

   21 to 24

146,568

166

1.17 (0.84 to 1.63)

150,692

727

1.07 (0.92 to 1.25)

   25 to 29

158,816

183

1.18 (0.84 to 1.66)

163,749

888

1.22 (1.05 to 1.43)

   30 to 34

51,266

50

0.99 (0.65 to 1.50)

52,784

281

1.22 (1.01 to 1.47)

   ≥ 35

16,091

36

2.18 (1.36 to 3.49)

16,644

94

1.27 (0.99 to 1.65)

   P trend

  

0.09

  

0.002

   P for homogeneitya

  

0.82

   

Duration of breastfeeding, months

      

   Never

127,851

151

Reference

131,753

688

Reference

   < 6

113,869

132

0.97 (0.77 to 1.22)

117,111

571

0.94 (0.84 to 1.05)

   6-11

73,115

82

0.93 (0.71 to 1.22)

75,315

402

1.05 (0.93 to 1.19)

   12-23

67,172

79

1.03 (0.77 to 1.36)

69,112

352

1.02 (0.90 to 1.17)

   ≥ 24

37,699

35

0.87 (0.59 to 1.29)

38,744

180

0.99 (0.84 to 1.18)

   P trend

  

0.70

  

0.56

   P for homogeneitya

  

0.56

   
  1. RRs are from multivariable Cox proportional hazards regression models using age (in days) as the time metric and stratified by age (in years) with the adjustment for race, family history of breast cancer in a first degree relative, age at menarche, HT use, body mass index. aHomogeneity in risk estimates between breast CIS and invasive breast cancer. bCompared with nulligravid women. cAdditionally, number of full-term pregnancies, age at first full-term pregnancy, and duration of breastfeeding mutually adjusted among parous women. dAdditionally excluded parous women who were missing information on breastfeeding or age at first full-term pregnancy. Abbreviations: CIS, carcinoma in situ; RR, relative risk; CI, confidence interval; HT, hormone therapy.
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Breast Cancer Research

ISSN: 1465-542X

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