Is gestational age an independent variable affecting uterine scar rupture rates?

OBJECTIVE: To evaluate the influence of gestational age on uterine scar rupture.

METHODS: This was a population-based study of data from Cardiff Births Survey over a 10-year (1990-1999) period. Women with only one previous lower segment caesarean section with singleton uncomplicated pregnancy of 37 or more week's gestation, undergoing trial of vaginal delivery were included. SPSS version 10 was used for statistical analysis. Mann-Whitney, Fisher's exact test and Chi-square tests were used wherever appropriate. Odds ratio (OR) with confidence intervals (CI) was used to quantify the risk. Potential confounding by other factors was controlled using logistic regression and corrected odds ratios with 95% confidence intervals were calculated. The data was analysed separately for induced and spontaneous labours. Primary outcome measure assessed was uterine scar rupture rate. Secondary outcome measures were repeat caesarean section rates, maternal and perinatal mortality and morbidity.

RESULTS: Total sample number was 1620. Eighty percent (n = 1301) of the population went into spontaneous labour and 20% (n = 319) were induced. Successful trial of vaginal birth was accomplished in 60% and trial of scar after estimated date of delivery did not alter this outcome significantly (39.1% versus 43.6%, p > 0.05). We noted an overall scar rupture rate of 0.9% (n = 14) and caesarean section rate of 40.4% (n = 654). Scar rupture rates significantly increased in women who underwent trial of labour after estimated date of delivery (p < 0.001, OR 6.3, CI 1.9-20.2) without a corresponding increase in caesarean section, maternal and perinatal morbidity figures. The influence of gestational age on scar rupture persisted even after controlling for other confounding factors such as birth weight, induction of labour and BMI (corrected OR 1.9, CI 1.1-3.5).

CONCLUSIONS: The overall incidence of scar rupture and success of trial of scar after previous caesarean section in our population was similar to that quoted in the literature. Previous evidence has suggested that it is safe for these women to exceed 40 weeks gestation but our data do not support this.