| Summary: | In England and Wales, the Caesarean section (CS) rate is 21.5%, ranging from 6% to 66% between maternity units. The impact of a high CS rate on women's health and NHS resources is not clear. Case-mix differences should be taken into account to enable valid comparisons and exploration of factors contributing to this variation. An understanding of these factors is important to ensure quality of obstetric care. The aim of this thesis was to explore the variation in CS rates between maternity units and evaluate the impact of (I) case-mix and (ii) women's birth preferences using National Sentinel Caesarean Section Audit (NSCSA) data. Summary of NSCSA data: Phase 1 (01.05.2000 to 31.07.2000) • Information on 150,139 women giving birth in 216 maternity units in England and Wales. Variables collected include age, ethnicity, parity, number of previous CS, mode of onset of labour, gestation, presentation, mode of delivery and birth weight. Phase 2 (01.12.2000 to 31 .01 .2001) • Survey of 2,475 pregnant women from 40 selected maternity units. Variables include preferred type of birth. Case-mix data were also collected for all 32,536 women giving birth in these maternity units. The relationship between case-mix variables and CS (i) before labour and (ii) during labour was demonstrated using logistic regression. Using tese results, standardised CS rates were calculated for individual maternity units. Using meta-analytical techniques, the amount of variation in CS rates explained by case-mix adjustment was quantified. Data on preferred type of birth were available for 7% of women in Phase 2. Therefore various techniques for handling 'missing data' including multiple imputations were researched and applied to these data. Key findings: . The association between CS and case-mix variables vary for CS before labour and CS during labour. The odds of CS (before and in labour) increase with maternal age. Women from ethnic minority groups have lower odds of CS before labour, and increased odds of CS in labour. Women with a previous vaginal delivery have lower odds of CS, although the magnitude of this for CS before and in labour is markedly different. . Adjustment for case-mix explained 34% of the variance in CS rates between maternity units. • Adjustment for case-mix differences and women's birth preferences explained 45% of the variance in CS rates between maternity units in England and Wales. 3
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