Adjusted (maternal characteristics (MC)) Adjusted (maternal characteristics (MC)) Adjusted (MC + delivery mode (DM)) Adjusted (MC + delivery mode (DM)) Adjusted (MC + DM + birth weight) Adjusted (MC + DM + birth weight)
RR (95%CI) RR (95%CI) RR (95%CI)
Year of delivery 2005–2008 1.00 Reference 1.00 Reference 1.00 Reference
2009–2012 0.94 0.92–0.97 0.96 0.94–0.98 0.96 0.93–0.98
2013–2016 0.92 0.90–0.95 0.97 0.94–0.99 0.96 0.94–0.99
Maternal age (years) <20 0.45 0.40–0.50 0.50 0.45–0.56 0.49 0.44–0.55
20–24 0.67 0.65–0.70 0.72 0.69–0.74 0.70 0.68–0.73
24–29 1.0 Reference 1.00 Reference 1.00 Reference
30–34 1.24 1.21–1.27 1.18 1.15–1.21 1.20 1.17–1.23
34–39 1.29 1.25–1.34 1.18 1.14–1.22 1.23 1.19–1.27
≥40 1.23 1.15–1.32 1.09 1.01–1.17 1.18 1.10–1.26
Parity 1 3.13 3.05–3.21 2.47 2.40–2.53 2.96 2.88–3.04
2 1.0 Reference 1.00 Reference 1.00 Reference
≥3 0.27 0.25–0.29 0.29 0.27–0.31 0.27 0.26–0.29
Smoking Yes 0.64 0.60–0.68 0.65 0.61–0.69 0.74 0.70–0.79
No 1.0 Reference 1.00 Reference 1.00 Reference
Not known 0.95 0.88–1.03 0.96 0.88–1.04 0.96 0.89–1.04
Body Mass Index (kg/m2) <18.5 1.12 1.05–1.20 1.11 1.04–1.18 1.30 1.22–1.38
20–24.9 1.0 Reference 1.00 Reference 1.00 Reference
25–29.9 1.07 1.04–1.10 1.06 1.04–1.09 0.97 0.94–0.99
≥30 1.04 1.04–1.10 1.04 1.00–1.08 0.90 0.87–0.94
Not known 1.01 0.95–1.08 1.02 0.95–1.08 0.98 0.92–1.04
Maternal height (cm) <155 1.34 1.26–1.42 1.24 1.17–1.31 1.72 1.62–1.82
155–164 1.11 1.08–1.13 1.07 1.04–1.09 1.23 1.21–1.26
165–174 1.0 Reference 1.00 Reference 1.00 Reference
≥175 0.92 0.88–0.95 0.94 0.91–0.98 0.82 0.79–0.85
Not known 1.03 0.93–1.14 1.01 0.92–1.12 1.09 0.99–1.21
Maternal education level ≤Primary/lower secondary 0.85 0.81–0.89 0.84 0.80–0.88 0.87 0.83–0.92
Upper secondary 0.88 0.85–0.90 0.87 0.85–0.89 0.87 0.85–0.90
<3 yrs higher education 0.92 0.89–0.95 0.92 0.89–0.95 0.92 0.90–0.96
≥3 yrs higher education 1.00 Reference 1.00 Reference 1.00 Reference
Not known 0.94 0.90–0.98 0.93 0.89–0.97 0.95 0.91–0.98
Maternal country of birth Nordic countries 1.00 Reference 1.00 Reference 1.00 Reference
Europe/USA/ Canada/Australia/NZ 0.91 0.84–0.98 0.91 0.84–0.98 0.92 0.86–0.99
Other 1.21 1.17–1.25 1.20 1.16–1.24 1.31 1.26–1.35
Delivery mode Non-instrumental 1.00 Reference
VE 2.79 2.73–2.86
Forceps 4.27 3.86–4.72