Unexplained variation in hospital caesarean section rates.

OBJECTIVES To assess recent hospital caesarean section (CS) rates in New South Wales, adjusted for case mix; to quantify the amount of variation that can be explained by case mix differences; and to examine the potential impact on the overall CS rate of reducing variation in practice. DESIGN AND SETTING Population-based record linkage study of births in 81 hospitals in New South Wales, 2009-2010, using the Robson classification to categorise births, and multilevel logistic regression to examine variation in hospital CS rates within Robson groups. MAIN OUTCOME MEASURES Hospital CS rates. RESULTS The overall CS rate was 30.9%, ranging from 11.8% to 47.4% (interquartile range, 23.9%-33.1%) among hospitals. The three groups contributing most to the overall CS rate all comprised women with a single cephalic pregnancy who gave birth at term, including: those who had had a previous CS (36.4% of all CSs); nulliparous women with an elective delivery (prelabour CS or labour induction, 23.4%); and nulliparous women with spontaneous labour (11.1%). After adjustment for case mix, marked unexplained variation in hospital CS rates persisted for: nulliparous women at term; women who had had a previous CS; multifetal pregnancies; and preterm births. If variation in practice was reduced for these risk-based groups by achieving the "best practice" rate, this would lower the overall rate by an absolute reduction of 3.6%, from 30.9% to 27.3%. CONCLUSION Understanding hospital heterogeneity in performing CS and implementing evidence-based practices may result in improved maternity care. We have identified five risk-based groups as priority targets for reducing practice variation in CS rates.