BACKGROUND: There is an ongoing debate concerning which guidelines and monitoring tools are most beneficial for assessing labour progression, to help prevent use of intrapartum caesarean section (ICS). The WHO partograph has been used for decades with the assumption of a linear labour progression; however, in 2010, Zhang introduced a new guideline suggesting a more dynamic labour progression. We aimed to investigate whether the frequency of ICS use differed when adhering to the WHO partograph versus Zhang's guideline for labour progression. METHODS: We did a multicentre, cluster-randomised controlled trial at obstetric units in Norway, and each site was required to deliver more than 500 fetuses per year to be eligible for inclusion. The participants were nulliparous women who had a singleton, full-term fetus with cephalic presentation, and who entered spontaneous active labour. The obstetric units were treated as clusters, and women treated within these clusters were all given the same treatment. We stratified these clusters by size and number of previous caesarean sections. The clusters containing the obstetric units were then randomly assigned (1:1) to the control group, which adhered to the WHO partograph, or to the intervention group, which adhered to Zhang's guideline. The randomisation was computer-generated and was done in the Unit of Biostatistics and Epidemiology, Oslo University Hospital, Oslo, Norway, and investigators in this unit had no further involvement in the trial. Our study design did not enable masking of participants or health-care providers, but the investigators who were analysing the data were masked to group allocation. The primary outcome was use of ICS during active labour (cervical dilatation of 4-10 cm) in all participating women. The Labour Progression Study (LaPS) is registered with ClinicalTrials.gov, number NCT02221427. FINDINGS: Between Aug 1, 2014, and Sept 1, 2014, 14 clusters were enrolled in the LaPS trial, and on Sept 11, 2014, seven obstetric units were randomly assigned to the control group (adhering to the WHO partograph) and seven obstetric units were randomly assigned to the intervention group (adhering to Zhang's guideline). Between Dec 1, 2014, and Jan 31, 2017, 11?615 women were judged to be eligible for recruitment in the trial, which comprised 5421 (46·7%) women in the control group units and 6194 (53·3%) women in the intervention group units. In the control group, 2100 (38·7%) of 5421 women did not give signed consent to participate and 16 (0·3%) women abstained from participation. In the intervention group, 2181 (35·2%) of 6194 women did not give signed consent to participate and 41 (0·7%) women abstained from participation. 7277 (62·7%) of 11?615 eligible women were therefore included in the analysis of the primary endpoint. Of these women, 3305 (45·4%) participants were in an obstetric unit that was randomly assigned to the control group (adhering to the WHO partograph) and 3972 (54·6%) participants were in an obstetric unit that was randomly assigned to the intervention group (adhering to Zhang's guideline). No women dropped out during the trial. Before the start of the trial, ICS was used in 9·5% of deliveries in the control group obstetric units and in 9·3% of intervention group obstetric units. During our trial, there were 196 (5·9%) ICS deliveries in women in the control group (WHO partograph) and 271 (6·8%) ICS deliveries in women in the intervention group (Zhang's guideline), and the frequency of ICS use did not differ between the groups (adjusted relative risk 1·17, 95% CI 0·98-1·40; p=0·08; adjusted risk difference 1·00%, 95% CI -0·1 to 2·1). We identified no maternal or neonatal deaths during our study. INTERPRETATION: We did not find any significant difference in the frequency of ICS use between the obstetric units assigned to adhere to the WHO partograph and those assigned to adhere to Zhang's guideline. The overall decrease in ICS use that we observed relative to the previous frequency of ICS use noted in these obstetric units might be explained by the close focus on assessing labour progression more than use of the guidelines. Our results represent an important contribution to the discussion on implementation of the new guideline. FUNDING: ?stfold Hospital Trust.