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

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.