Statistics
Risk Ratios (RR) and Adjusted Risk Ratios (aRR) for sphincter rupture were obtained using modified Poisson Regression analyses (SPSS version 25). When specified, adjustments were made for year of delivery (classes 2005–2008, 2009–2012, 2013–2016) and maternal characteristics: Maternal age (classes <20, 20–24.9, 25–29.9, 30–34.9, 35–39.9, or ≥40 years), parity (1, 2, or ≥3 children), BMI classes (<18.5, 18.5–24.9, 25–29.9, ≥30, or not known), smoking (yes, no, or not known), maternal height classes (<155, 155-164, 165-174, ≥175 cm, or not known), maternal country of birth (Nordic countries, other European countries/USA/Canada/Australia/New Zeeland, others, or not known), and maternal educational level (≤primary/lower secondary (1-9 years), upper secondary (10-12 years), ≤3 years higher education, ≥3 years higher education, or not known). When specified, adjustments were also made for delivery mode (vaginal non-instrumental, vacuum extraction, or forceps), and/or infant birth weight (<2500, 2500-2999, 3000-3499, 3500-3999, 4000-4499, ≥4500g). Findings with p-values <0.05 were regarded as statistically significant.