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Objective: To evaluate the discrepancy between historical and more recent descriptions of the first stage of labour by testing whether the statistical techniques used recently (repeated‐measures polynomial and interval‐censored regression) were appropriate for detection of periods of rapid acceleration of cervical dilatation as might occur at the time of transition from a latent to an active phase of labour. Design and setting: A simulation study using regression techniques. Sample: We created a simulated data set for 500 000 labours with clearly defined latent and active phases using the parameters described by Friedman. Additionally, we created a data set comprising 500 000 labours with a progressively increasing rate of cervical dilatation. Methods: Repeated‐measures polynomial regression was used to create summary labour curves based on simulated cervical examinations. Interval‐censored regression was used to create centimetre‐by‐centimetre estimates of rates of cervical dilatation and their 95th centiles. Main outcome measures: Labour summary curves and rates of cervical dilatation. Results: Repeated‐measures polynomial regression did not detect the rapid acceleration in cervical dilatation (i.e. acceleration phase) and overestimated lengths of labour, especially at smaller cervical dilatations. There was a two‐fold overestimation in the mean rate of cervical dilatation from 4 to 6 cm. Interval‐censored regression overestimated median transit times, at 4‐ to 5‐cm cervical dilatation or when cervical examinations occurred less frequently than 0.5‐ to 1.5‐hourly. Conclusion: Repeated‐measures polynomial regression and interval‐censored regression should not be routinely used to define labour progress because they do not accurately reflect the underlying data. Repeated‐measures polynomial and interval‐censored regression techniques are not appropriate to model first stage of labour. Repeated‐measures polynomial and interval‐censored regression techniques are not appropriate to model first stage of labour. [ABSTRACT FROM AUTHOR]