How much variation in CS rates can be explained by case mix differences?

OBJECTIVE: To quantify the amount of variation in caesarean section (CS) rates between maternity units explained by case mix differences. DESIGN: Cross-sectional study. SETTING: All 216 maternity units in England and Wales. POPULATION: Women giving birth at these maternity units between May and July 2000. METHODS: Logistic regression models were developed to investigate the relationship between case mix characteristics, and odds of (i) CS before labour, (ii) CS in labour. Using these results, overall CS rates standardised for case mix were calculated for each maternity unit. Random-effects meta-analysis was used to examine heterogeneity between maternity units. MAIN OUTCOME MEASURES: CS before labour and CS during labour. RESULTS: Adjustment for case mix differences between maternity units explained 34% of the variance in CS rates. Odds of CS (before and in labour) increased with maternal age. Women from ethnic minority groups had lower odds of CS before labour, and increased odds of CS in labour. Women with a previous vaginal delivery had lower odds of CS, although the magnitude of this for CS before and in labour is markedly different. CONCLUSIONS: Case mix adjustment is important to enable understanding of the factors that influence the CS rate. These include organisational and staffing levels as well as women's preferences for childbirth and clinician's attitudes. An understanding of how these factors influence the CS rate is essential for evaluation of quality and appropriateness of obstetric care provided to women.