RESUMEN
INTRODUCTION: Maternal age is an established risk factor for cesarean section; epidural analgesia and oxytocin augmentation may modify this association. We investigated the effects and interactions of oxytocin augmentation, epidural analgesia and maternal age on the risk of cesarean section. MATERIAL AND METHODS: In all, 416 386 nulliparous women with spontaneous onset of labor, ≥37 weeks of gestation and singleton infants with a cephalic presentation during 2000-2011 from Norway and Denmark were included [Ten-group classification system (Robson) group 1]. In this case-control study the main exposure was maternal age; epidural analgesia, oxytocin augmentation, birthweight and time period were explanatory variables. Chi-square test and logistic regression were used to estimate associations and interactions. RESULTS: The cesarean section rate increased consistently with advancing maternal age, both overall and in strata of epidural analgesia and oxytocin augmentation. We observed strong interactions between maternal age, oxytocin augmentation and epidural analgesia for the risk of cesarean section. Women with epidural analgesia generally had a reduced adjusted odds ratio when oxytocin was used compared with when it was not used. In Norway, this applied to all maternal age groups but in Denmark only for women ≥30 years. Among women without epidural, oxytocin augmentation was associated with an increased odds ratio for cesarean section in Denmark, whereas no difference was observed in Norway. CONCLUSIONS: Oxytocin augmentation in nulliparous women with epidural analgesia is associated with a reduced risk of cesarean section in labor with spontaneous onset.