Table 3 The proportion of preterm deliveries in Norway, 1995–2004, presented according to gestational age groups, selected risk factors and the rate and absolute number of the reference population.

Deliveries (n)

53044

54058

53023

52113

58098

58024

55519

54545

55791

56184

22–27 w

0.22

0.23

0.23

0.25

0.28

0.32

0.29

0.32

0.31

0.26

< 0.001(I)⁵

28–31 w

0.64

0.67

0.61

0.68

0.73

0.70

0.68

0.70

0.74

0.63

0.14

32–36 w

5.1

4.9

5.1

5.1

5.1

5.2

5.4

5.5

5.2

5.5

< 0.001(I)

< 37 w

6.0

5.8

5.9

6.0

6.2

6.2

6.4

6.5

6.2

6.4

< 0.006(I)

Women not born in Norway

9.9

10.2

10.7

11.4

12.4

13.7

13.9

15.1

16.0

16.5

< 0.001(I)

IVF or ICSI

0.7

0.7

0.8

1.0

1.4

1.5

1.5

1.7

1.8

1.9

< 0.001 (I)

Multiples

1.6

1.5

1.6

1.6

1.8

1.8

1.8

1.9

1.9

1.9

< 0.001(I)

Primiparous ¹

40.9

40.9

40.4

41.0

42.9

41.0

40.9

41.1

41.6

42.2

< 0.001(I)

Iatrogenic delivery ²

15.9

16.1

15.8

15.2

16.6

16.1

17.1

17.0

18.0

18.5

< 0.001(I)

Age < 20

2.7

2.7

2.6

2.6

2.6

2.6

2.6

2.5

2.2

2.0

< 0.001(R)⁶

Age ≥35

12.0

12.2

13.1

13.5

13.6

14.2

15.1

15.4

16.4

17.0

< 0.001(I)

Reference population ³ (% of all births)

32.0

32.1

31.3

31.1

31.9

30.0

29.7

29.3

29.3

29.7

< 0.001(R)

Reference population ³ (n)

16971

17354

16619

16202

18516

17423

16511

15972

16331

16675