Table 4

The proportion of preterm deliveries in the reference population1 in Sweden (S), Denmark (D) and Norway (N), 1995–2004, presented according to gestational age groups.
Gestational weeks
 Country
 1995
 1996
 1997
 1998
 1999
 2000
 2001
 2002
 2003
 2004
 p-value for trend2
22–27
 S
 0.18
 0.22
 0.22
 0.22
 0.27
 0.15
 0.19
 0.20
 0.25
 0.26
 0.13

D
 0.19
 0.22
 0.23
 0.19
 0.20
 0.19
 0.22
 0.13
 0.27
 0.21
 0.83

N
 0.18
 0.20
 0.21
 0.20
 0.15
 0.27
 0.18
 0.20
 0.19
 0.14
 0.65
p for homogeneity of trends: 0.37
28–31
 S
 0.55
 0.69
 0.56
 0.60
 0.64
 0.54
 0.62
 0.62
 0.59
 0.57
 0.80

D
 0.66
 0.67
 0.62
 0.67
 0.65
 0.57
 0.61
 0.63
 0.59
 0.66
 0.48

N
 0.53
 0.43
 0.40
 0.61
 0.38
 0.34
 0.35
 0.45
 0.35
 0.26
 < 0.001(R)3
p for homogeneity of trends: 0.0034
32–36
 S
 5.5
 5.3
 5.1
 5.2
 5.4
 5.5
 5.0
 5.3
 5.2
 5.6
 0.82

D
 4.5
 5.0
 4.3
 4.8
 5.0
 5.0
 5.2
 5.4
 5.6
 5.4
 < 0.001(I)4

N
 4.7
 4.5
 4.6
 5.1
 4.3
 4.4
 4.6
 4.7
 4.4
 4.4
 0.18
p for homogeneity of trends: < 0.001
< 37
 S
 6.3
 6.2
 5.9
 6.0
 6.3
 6.2
 5.9
 6.1
 6.1
 6.5
 0.67

D
 5.3
 5.9
 5.2
 5.6
 5.9
 5.7
 6.0
 6.2
 6.5
 6.3
 < 0.001(I)

N
 5.4
 5.1
 5.2
 5.9
 4.8
 5.0
 5.2
 5.3
 5.0
 4.8
 0.018 (R)
p for homogeneity of trends: < 0.001

1 Reference population: 20 – 35-years-old primiparas, born in the country in which delivery occurs, with singleton, spontaneously conceived pregnancies.

2 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.

3(R) = Reduction

4(I) = Increase

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