RESUMEN
OBJECTIVE: To evaluate the effects of maternal age, induction of labour, epidural analgesia and birth weight on mode of delivery in nulliparous women with a singleton pregnancy and cephalic presentation at > or =36 weeks gestation, and to describe how these factors and their influence have changed over a 17-year period from 1989 to 2005. STUDY DESIGN: The study was conducted in the obstetric department of a university teaching hospital in Ireland. Of 45,647 women delivered, 14,867 were nulliparous with a singleton pregnancy and cephalic presentation and undergoing labour at > or =36 weeks gestation, and were included in the study. The main outcome measures were the influence of maternal age, induction of labour, epidural analgesia and birth weight on the mode of delivery. Multinomial logistic regression analysis for type of delivery and the associated explanatory variables and trend analysis of these variables were performed. RESULTS: There was a significant progressive increase in both unplanned abdominal delivery and instrumental vaginal delivery, with advancing maternal age. Induction of labour increased the risk of unplanned abdominal delivery (OR 1.92; 95% CI 1.73-2.14). Epidural analgesia was associated with an increased risk of instrumental vaginal delivery (OR 4.68; 95% CI 4.18-5.25), and unplanned abdominal delivery (OR 2.29; 95% CI 1.98-2.66). Mothers of infants with birth weight > or =4.5 kg were less likely to be delivered by instrumental vaginal delivery (OR 0.60; 95% CI 0.41-0.88), than mothers delivering infants in the 2.50-4.49 kg birth weight category. Between 1989 and 2005 there was a significant increase in maternal age (P=0.0001), birth weight (P=0.042) and unplanned abdominal delivery rates (P=0.0004), and a reduction in instrumental vaginal delivery rates (P=0.0013). CONCLUSIONS: These data demonstrate that the increasing trend of unplanned abdominal delivery in nulliparous women with a singleton pregnancy and cephalic presentation may be partially explained by advancing maternal age, and other obstetric factors also play a significant role.