Table 1 Breast cancer events and person-years according to number of childbirths, age at first childbirth, and duration of latest pregnancy in the Danish and the Norwegian cohorta

From: Pregnancy duration and breast cancer risk

Cohort characteristic

The Danish cohort

The Norwegian cohort

Breast cancer events (%)

Persons-years in 1000s (%)

Breast cancer events (%)

Persons-years in 1000s (%)

 Number of childbirths

 0

8028 (13.1)

23,370 (50.7)

2880 (13.9)

13,861 (55.1)

 1

10,523 (17.1)

4418 (9.6)

2996 (14.4)

1850 (7.3)

 2

28,046 (45.7)

11,938 (25.9)

8582 (41.3)

5268 (20.9)

 3

11,468 (18.7)

4888 (10.6)

4639 (22.3)

2965 (11.8)

 4

2667 (4.3)

1175 (2.5)

1313 (6.3)

901 (3.6)

 ≥5

617 (1.0)

340 (0.7)

367 (1.8)

324 (1.3)

Age at first childbirth (years)

 Nulliparous

8028 (13.1)

23,370 (50.7)

2880 (13.9)

13,861 (55.1)

 <20

8538 (13.9)

4109 (8.9)

3021 (14.5)

2490 (9.9)

 20–21

9630 (15.7)

4535 (9.8)

3692 (17.8)

2682 (10.7)

 22–23

10,150 (16.5)

4569 (9.9)

3320 (16.0)

2221 (8.8)

 24–25

8956 (14.6)

3814 (8.3)

2957 (14.2)

1695 (6.7)

 26–27

6373 (10.4)

2531 (5.5)

1979 (9.5)

1018 (4.0)

 28–29

4087 (6.7)

1489 (3.2)

1247 (6.0)

579 (2.3)

 ≥30

5587 (9.1)

1712 (3.7)

1681 (8.1)

621 (2.5)

Duration of latest pregnancy (weeks) b

 Nulliparous

8028 (13.1)

23,370 (50.7)

2880 (13.9)

13,861 (55.1)

 20–27

34 (0.1)

12 (0.0)

29 (0.1)

16 (0.1)

 28–29

38 (0.1)

18 (0.0)

22 (0.1)

15 (0.1)

 30

33 (0.1)

14 (0.0)

20 (0.1)

13 (0.1)

 31

33 (0.1)

16 (0.0)

40 (0.2)

18 (0.1)

 32

54 (0.1)

26 (0.1)

44 (0.2)

25 (0.1)

 33

78 (0.1)

35 (0.1)

62 (0.3)

36 (0.1)

 34

101 (0.2)

54 (0.1)

92 (0.4)

61 (0.2)

 35

178 (0.3)

81 (0.2)

154 (0.7)

100 (0.4)

 36

360 (0.6)

163 (0.4)

262 (1.3)

170 (0.7)

 37

719 (1.2)

332 (0.7)

507 (2.4)

334 (1.3)

 38

1626 (2.7)

798 (1.7)

1254 (6.0)

792 (3.1)

 39

3091 (5.0)

1519 (3.3)

2959 (14.2)

1795 (7.1)

 40

6369 (10.4)

3062 (6.6)

3902 (18.8)

2523 (10.0)

 41

2846 (4.6)

1446 (3.1)

2877 (13.8)

1886 (7.5)

 ≥42

1288 (2.1)

685 (1.5)

1777 (8.6)

1222 (4.9)

 Missing duration of pregnancy in birth register

1493 (2.4)

662 (1.4)

638 (3.1)

471 (1.9)

 Childbirths registered in civil registersc

34,980 (57.0)

13,835 (30.0)

3217 (15.5)

1799 (7.1)

  1. aAll events and person-years from 10 years after latest childbirth
  2. bPregnancies registered to have lasted less than 20 weeks or more than 45 gestational weeks were also included in the analysis as separate categories (see Statistical analyses), but constituted combined only <0.01% and 0.13% of observation time in Denmark and Norway, respectively
  3. cChildbirths registered in the civil registration systems, but not in the Birth Registers. Predominantly childbirths before January 1, 1978 in Denmark and January 1, 1967 in Norway. After these dates only 3.34% and 3.85% of childbirths are not reported in the Medical Births Registers, in Denmark and Norway, respectively
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