Objective: To validate an intrapartum Cesarean Section Classification System (ICSCS). Design: Nationwide prospective observational study. Setting: Twenty-five Norwegian maternity units Population or Sample: Singleton cephalic pregnancies with spontaneous or induced labour at ≥ 37 weeks gestation delivering February-August 2017. Methods:. After training of all collaborators, Cesarean section (CS) after spontaneous or induced labour were classified based on fetal status, dynamic progress in labour, use of oxytocin, frequency of contractions and linked to denominator data collected and centralized by the Norwegian Medical Birth Registry. Main Outcome Measures: Cohens kappa as measure of agreement for correct application of the classification. Prevalence of the different groups using the ICSCS within the Ten Group Classification System (Robson groups). Results: Of 49 trained experts, 40 (82%) had a κ >0.6 indicating good or very good level of agreement when the classification was applied. A total of 1425 CS were classified: CS classified as fetal indication (no oxytocin) was more common in induced (Group 2a, 4a, 5b) compared to spontaneous labours (Group 1, 3, 5a). CS classified as dystocia related to inefficient uterine action and poor response to oxytocin occurred more often in induced as compared to spontaneous labours. The prevalence of CS classified as dystocia with efficient uterine action (malposition or cephalopelvic disproportion) was low in all Robson groups. Conclusions: The ICSCS was successfully validated in a national study. It may become a valuable objective tool for analyzing the management of labour and explaining differences in the prevalence of CS between different groups of women.