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.

Norway
 1995
 1996
 1997
 1998
 1999
 2000
 2001
 2002
 2003
 2004
 p-value for trend4
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)
Primiparous1
 40.9
 40.9
 40.4
 41.0
 42.9
 41.0
 40.9
 41.1
 41.6
 42.2
 < 0.001(I)
Iatrogenic delivery2
 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 population3 (% 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 population3 (n)
 16971
 17354
 16619
 16202
 18516
 17423
 16511
 15972
 16331
 16675

P-values for trend are presented.

1 Primiparous: the index pregnancy resulted in delivery for the first time.

2 Induced delivery or caesarean section.

3 Reference population: 20 – 35-years-old primiparas, born in Norway, with singleton, spontaneously conceived pregnancies.

4 p-values obtained from simple logistic regression analyses with the dichotomized variables specified as outcome variables, and year of birth as the only independent variable.

5(I) = Increase

6(R) = Reduction

Morken et al. BMC Women's Health 2008 8:16   doi:10.1186/1472-6874-8-16