Reference population for international comparisons and time trend surveillance of preterm delivery proportions in three countries

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

ISSN: 1472-6874