For whom is the Caesarean section rate high?

To define a method for examining and comparing Caesarean section rates.Data on Caesarean section rates at National Women's Hospital for 1997 were analysed by two methods which adjust Caesarean section rates according to casemix: 1. standard nulliparae and 2. grouping women by factors influencing Caesarean section rates.The Caesarean section rate in New Zealand is rising and National Women's Hospital had a Caesarean section rate of 22.3% in 1997. The instrumental vaginal delivery rate was 13.1%. Our 1997 data were analysed for casemix -- standard nulliparae and dividing women into six distinct groups. The Caesarean section rate for standard nulliparae was 19.2%. Nulliparae had an increased induction of labour rate compared to multiparae (29.1% versus 22.9%, p < 0.001). The Caesarean section rate was increased in association with induction of labour for nulipare (26.0% versus 13.1%, p < 0.001), multiparae with unscarred uteri, (5.4% versus 2.5%, p < 0.001) and primiparae with previous caesareans (36.8% versus 30.4%, p = 0.02)We suggest our second method of grouping women by the most important factors influencing intervention rates should be used by obstetric units for benchmarking and internal audit purposes.