Table 2

Population characteristics by study area

Maternal and pregnancy factors	Alberta Canada N=256 137%	NSW Australia N=732 288%	WA Australia N=149 624%	Denmark N=645 993%	Norway N=456 353%	Scotland N=531 622%	Sweden N=913 779%	Massachusetts USA N=762 723%
Study period	2002–2007	1998–2006	2000–2005	1997–2006	1999–2006	1997–2006	1997–2006	1998–2007
Maternal age (years)
<20	5.8	4.3^*	4.6^*	1.6^*	2.4	8.3^*	1.8	6.5
20–34	79.5	76.7^*	75.2^*	82.6	81.7	74.6^*	80.1	71.5
≥35	14.7	19.0^†	20.2^†	15.8^†	15.9^†	17.1^†	18.1^†	22.0
Nullipara	44.2^†	41.6^†	41.6^†	42.9^†	40.7^†	45.2^†	43.9^†	44.6
Multiple births	1.6^‡	1.6^†	1.7	1.5^†	1.9	1.5	1.5^*	2.3
Diabetes (any)	4.0	5.4^†	4.3	1.7^†	1.5^†	0.8	1.3	5.3
Gestational diabetes	3.5	5.0^†	3.8	1.3	0.8^†	0.5	0.9	4.4
Pre-existing DM	0.5	0.4^†	0.6	0.4	0.7^†	0.4	0.4^†	1.0
Chronic hypertension	0.4	1.2^†	1.1	0.4^†	0.6^*	0.3	0.5^†	1.6
Smoking	NA	17.4^*	19.3^*	20.6^* ^§	18.6^* ^¶	24.8^*	10.7^* ^††	12.3
Smoking	NA	17.4^*	19.3^*	20.6^* ^§	13.1^* ^**	24.8^*	10.7^* ^††	12.3
Induction of labour	21.4^*	24.6^†	29.2^†	12.4^†	12.0^†	25.8	17.8^†	20.1
Mode of delivery
Normal vaginal	61.4^*	64.8^*	57.6	75.7^*	77.3	65.3^*	76.8^*	66.5
Instrumental	13.0^*	10.4^*	12.4	7.2^†	8.1^†	12.5	7.5	6.1
Caesarean section	25.6^†	24.7^†	30.0^†	17.1^†	14.8^†	22.2^†	15.7^†	27.4
Term births
≥40 weeks	40.1	49.6^*	43.6^*	54.1^*	53.6^*	54.4^*	53.5^*
37–39 weeks	51.2	43.9^†	48.8^†	39.4^†	40.0^†	38.6^†	40.8^†
Preterm births (all)	8.4	6.5^†	7.6	6.5^†	6.4	7.0	5.5	9.5
Elective—see below	2.5	2.5^†	3.6^†	1.3^†	2.8	0.6^§§	1.6	NA
Spontaneous	5.9	4.0	4.0	5.2	3.6	6.4	3.9^†
Outcomes
Any pregnancy hypertension	6.0^*	8.8^*	9.1^*	3.6	5.8	5.9^*	3.9^*	7.0^†
Pre-eclampsia	1.4^†	3.1^*	2.9^*	2.7^*	4.0^*	2.2^*	2.9^*	3.3^†
Pre-eclampsia ≤34 weeks	0.7^‡‡	0.3	0.6	0.3	0.4	0.3	0.4	0.6

Percentages may not total 100 because of missing data.
* Decreasing over the study period, χ² for trend p<0.01.
† Increasing over the study period, χ² for trend p<0.01.
‡ Alberta—multiple birth data available for 98.5% of women.
§ Denmark—smoking data available for 96% of women.
¶ Norway—daily smoking in the first trimester (available for 83% of women).
** Norway—daily smoking in the last trimester (available for 78% of women).
†† Sweden—smoking data available for 93% of women.
‡‡ Alberta—gestational age available for 2004–2007.
§§ Scotland—elective caesarean sections only.