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Objective: To study whether there are significant differences in the rate of obstetric anal sphincter injuries (OASIS) between the different sized delivery units in Finland.

Study design: The study was performed as a population based registry study in Finland, including all births (294 725) between 2006 and 2010. All the Finnish delivery units (34) were categorized by the number of annual deliveries and the OASIS rate was then compared between the different sized delivery units using a logistic regression analysis adjusting for maternal age and parity. The Robson ten group classification was used for more accurate comparison.

Results: The OASIS rate was significantly elevated, both in the largest units with 5000 annual deliveries or more (OR 1.46, 95% CI 1.11-1.92) and in the smallest units with less than 500 annual deliveries (OR 1.33, 95% CI 1.22-1.45). In the Robson’s group 1 (primiparous, single cephalic term pregnancy, spontaneous labour) the risk for OASIS was the highest in the largest units (OR 1.44, 95% CI 1.28-1.61) while in the Robson’s group 3 (multiparous, single cephalic term pregnancy, spontaneous labour) the highest risk was found in the smallest units (OR 2.90, 95% CI 1.68-5.02).

Conclusions: There is significant inter-hospital variation in OASIS rates suggesting significant differences in obstetric practices. Robson’s ten group classification should be used to enhance the inter-hospital comparison. (c) 2013 Elsevier Ireland Ltd. All rights reserved.