Table 5

The proportion of spontaneous preterm deliveries of the total number of 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
 0.10
 0.13
 0.13
 0.11
 0.11
 0.07
 0.12
 0.11
 0.12
 0.16
 0.34

D
 0.17
 0.18
 0.23
 0.21
 0.23
 0.21
 0.23
 0.13
 0.31
 0.24
 < 0.038(I)3

N
 0.15
 0.17
 0.17
 0.17
 0.13
 0.23
 0.15
 0.15
 0.16
 0.12
 0.334













28–31
 S
 0.22
 0.26
 0.24
 0.25
 0.28
 0.19
 0.23
 0.24
 0.28
 0.27
 0.44

D
 0.45
 0.44
 0.58
 0.64
 0.71
 0.58
 0.63
 0.71
 0.63
 0.72
 < 0.001(I)

N
 0.44
 0.36
 0.34
 0.52
 0.30
 0.29
 0.30
 0.36
 0.29
 0.22
 0.984













32–36
 S
 4.1
 3.8
 3.5
 3.6
 3.7
 3.9
 3.4
 3.7
 3.7
 3.9
 0.39

D
 3.8
 4.4
 4.3
 4.8
 5.2
 5.2
 5.2
 5.7
 5.8
 5.8
 < 0.001(I)

N
 4.0
 3.8
 3.9
 4.4
 3.6
 3.7
 3.9
 3.9
 3.7
 3.7
 0.294













< 37
 S
 4.4
 4.2
 3.9
 4.0
 4.1
 4.1
 3.8
 4.0
 4.1
 4.3
 0.64

D
 4.4
 5.0
 5.1
 5.7
 6.1
 6.0
 6.0
 6.5
 6.7
 6.8
 < 0.001(I)

N
 4.6
 4.3
 4.4
 5.0
 4.0
 4.2
 4.3
 4.5
 4.2
 4.0
 0.634

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 for 1995 to 2004.

3(I) = Increase

4Trend analyzed for 1999 to 2004, due to known registration changes in the Medical Birth Register of Norway in 1999.

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