As there is no universal definition of 'normal' labour, diagnosing prolonged labour is inherently difficult. Simple charts (partograms) are used to aid this process; however, there are little empirical data on which to base such charts. Therefore, we monitored 403 multiparous women, in the first stage of labour with uncomplicated term pregnancies, in a midwife-led unit. They had vaginal examinations every 2 hours, and we modelled rates of cervical dilatation for all women, either to the end of the first stage of labour or to the point of transfer for intervention. Overall, the median rate of dilatation was 1.9 cm/hour and the 5th percentile was 0.5 cm/hour. There was a weak positive correlation between initial dilatation and rate of dilatation (r= 0.2). Individualised computer-based partograms incorporating some of the factors that affect progress might prove useful in the future.