Table 3 Adjusted RRs for the association between pregnancy-related factors and the subtypes of invasive breast cancer

All women

Ever pregnant

   No

Reference

Reference

Reference

Reference

   Yes

0.87 (0.78 to 0.97)

0.99 (0.74 to 1.31)

0.81 (0.71 to 0.91)

1.04 (0.77 to 1.41)

P for homogeneity^a

0.42

0.12

Number of full-term pregnancy^b

   1

0.93 (0.80 to 1.09)

0.78 (0.51 to 1.20)

0.90 (0.75 to 1.06)

0.85 (0.55 to 1.33)

   2

0.88 (0.77 to 1.00)

1.15 (0.84 to 1.58)

0.80 (0.69 to 0.92)

1.19 (0.85 to 1.66)

   3

0.85 (0.74 to 0.98)

0.88 (0.62 to 1.26)

0.82 (0.70 to 0.95)

0.99 (0.68 to 1.43)

   ≥ 4

0.76 (0.65 to 0.89)

0.90 (0.61 to 1.34)

0.71 (0.59 to 0.85)

0.90 (0.59 to 1.37)

   P trend

0.0004

0.72

< 0.0001

0.85

P for homogeneity^a

0.33

0.17

Ever pregnant, incomplete pregnancies only or outcome of pregnancies unknown

Parous women only ^{c, d}

Number of full-term pregnancies

   1

Reference

Reference

Reference

Reference

   2

0.96 (0.83 to 1.11)

1.51 (1.01 to 2.25)

0.91 (0.77 to 1.08)

1.39 (0.92 to 2.10)

   3

0.95 (0.80 to 1.12)

1.18 (0.75 to 1.86)

0.95 (0.79 to 1.15)

1.17 (0.74 to 1.85)

   ≥ 4

0.85 (0.70 to 1.04)

1.24 (0.75 to 2.07)

0.84 (0.68 to 1.05)

1.08 (0.64 to 1.83)

   P trend

0.13

0.99

0.25

0.74

P for homogeneity^a

0.57

0.87

Age at first full-term pregnancy, years

   < 21

Reference

Reference

Reference

Reference

   21 to 24

1.04 (0.87 to 1.25)

1.37 (0.86 to 2.18)

1.04 (0.85 to 1.28)

1.31 (0.81 to 2.12)

   25 to 29

1.21 (1.01 to 1.45)

1.31 (0.82 to 2.10)

1.18 (0.97 to 1.45)

1.28 (0.79 to 2.07)

   30 to 34

1.17 (0.94 to 1.46)

1.71 (0.99 to 2.95)

1.23 (0.96 to 1.57)

1.66 (0.95 to 2.90)

   ≥ 35

77

1.37 (1.02 to 1.83)

1.21 (0.52 to 2.81)

1.36 (0.98 to 1.89)

1.07 (0.44 to 2.60)

   P trend

0.004

0.23

0.009

0.30

   P for homogeneity^a

0.96

0.92

Duration of breastfeeding, months

   Never

Reference

Reference

Reference

Reference

   < 6

0.98 (0.86 to 1.11)

0.97 (0.70 to 1.34)

1.03 (0.89 to 1.19)

0.96 (0.69 to 1.34)

   6 to 11

1.06 (0.92 to 1.23)

1.15 (0.81 to 1.63)

1.08 (0.91 to 1.27)

1.16 (0.81 to 1.67)

   12 to 23

1.04 (0.89 to 1.21)

1.02 (0.69 to 1.50)

1.08 (0.90 to 1.29)

1.02 (0.69 to 1.52)

   ≥ 24

1.02 (0.83 to 1.25)

1.00 (0.61 to 1.66)

1.00 (0.80 to 1.27)

1.06 (0.64 to 1.78)

   P trend

0.52

0.78

0.54

0.65

   P for homogeneity^a

0.98

0.88