The inclusion criteria for the study were that a woman had previously had one caesarean section and was delivered in her current pregnancy by a means other than planned caesarean section. We excluded women with more than one previous caesarean, those with multiple gestations, those delivering before 37 weeks' gestation, and those delivering beyond 43 weeks' gestation. We also excluded cases in which the infant died from causes other than intrapartum uterine rupture.
Data sources—The Scottish Morbidity Record (SMR2) collects information on clinical and demographic characteristics and outcomes for all patients discharged from Scottish maternity hospitals. The register is subjected to regular quality assurance checks and has been > 99% complete since the late 1970s.3 The register collects both specific obstetric data and up to six ICD-9 (international classification of diseases, ninth revision) or ICD-10 (10th revision) diagnostic codes relating to the admission. In 1996-7 a quality assurance analysis comparing 1414 records with the clinical notes showed that the register was free from significant errors in > 98% of records in all the specific fields used in the present analysis. Exceptions were postcode (94.0%), height (96.2%), estimated gestation (94.4%), and method of induction of labour (93.6%). The previous caesarean section field was 99.7% accurate. ICD diagnostic codes were found to be 80-90% accurate for the first four diagnoses and 70-80% accurate for the remainder (Jim Chalmers, personal communication). SMR2 records were linked to records from the Scottish Stillbirth and Infant Death Survey. This national register routinely classifies all perinatal deaths in Scotland. Coding of the cause of death is performed by a single medically qualified individual, and the survey is described in detail elsewhere.4
Definitions—We defined trial of labour as a singleton delivery at term by a means other than planned caesarean section in women with only one previous caesarean delivery. The definition of perinatal death due to uterine rupture was that the obstetric cause of death was coded as "mechanical" under the modified Wigglesworth classification5 and that the ICD-9 diagnostic code for intrapartum uterine rupture (665.1) was listed under the specific diagnoses. Intrapartum uterine ruptures that did not result in perinatal death were identified with ICD-9 and ICD-10 diagnostic codes 665.1 and O711, respectively, from the diagnostic fields in the SMR2 record related to hospital discharge after delivery. Hospital throughput was defined as the total number of births recorded in the SMR2 database for the given hospital over the given year. Hospital throughput was categorised into above or below the median (3000 births). Other maternal characteristics were defined as previously described.2
Statistical analyses—We summarised continuous variables with medians and interquartile ranges and used the Mann-Whitney U test for comparisons between groups and Fisher's exact test for univariate comparisons of dichotomous data. P values for all hypothesis tests were two sided. Multivariate analysis was performed using logistic regression analysis. The significance of interaction terms was assessed with the likelihood ratio test. The goodness of fit of models was assessed with the Hosmer and Lemeshow test based on tenths of probability.6 Because of the rarity of the event we used exact logistic regression to model the risk of perinatal death due to uterine rupture.7 When we treated annual number of births as a continuous variable in the exact model, we rounded it to the nearest 50 to make the model computationally feasible. All statistical analyses were performed with the Stata software package version 8.2 (StataCorp, College Station, TX), except for exact logistic regression, which was performed with LogXact version 5.0 (Cytel Software Corporation, Cambridge, MA).
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