Background: One of the major complications related to delivery is labor dystocia, or an arrested labor progress. Many dystocic deliveries end vaginally after administration of oxytocin, but a large numbers of women with labor dystocia will undergo a long and unsafe parturition. As a result of the exertion required in labor, the uterus produces lactate. The uterine production of lactate is mirrored by the level of lactate in amniotic fluid (AFL). Objectives: To evaluate whether the level of AFL, analysed in a sample of amniotic fluid collected vaginally at arrested labor when oxytocin was needed, could predict labor outcome in nulliparous deliveries. Methods: A prospective multicentre study including 3000 healthy primiparous women all with a singleton pregnancy, gestational age 37 to 42 weeks and no maternal/fetal chronic and/or pregnancy-related conditions. A spontaneous onset of labor, regular contractions and cervical dilation >= 3 cm were required before the women were invited to take part in the study. Results: AFL, analysed within 30 minutes before augmentation, provides information about delivery outcome. Sensitivity for an acute cesarean section according to high (>= 10.1 mmol/l) or low (< 10.1mmol/l) AFL values was 39.0% (95% CI; 27-50), specificity 90.3% (95% CI; 87-93) PPV 37.3% (95% CI; 27-48) and NPV was 91.0% (95% CI; 88-93). The overall percentage of correct predictions of delivery outcome when the AFL level was used was 83.7%. Deliveries with a high AFL-level correlated with delivery time > 12h (p = 0.04), post-partum fever (> 38 degrees C, p = 0.01) and post-partum haemorrhage > 1.5L (p = 0.04). Conclusion: The AFL is a good predictor of delivery outcome in arrested nulliparous deliveries. Low levels of AFL may support the decision to continue a prolonged vaginal labor by augmentation with oxytocin. A high level of AFL correlates with operative interventions and post-partum complications.