The observational study design has limitations when compared with a randomised controlled trial. However, it can be doubted whether an ethical committee would permit a deliberate exposure to the degree and risks of hyponatraemia observed in the present study. Analysis would have benefited from hourly registration of fluid intake and urine output. This option was considered, but although desirable, the work load imposed could have jeopardised the realisation of the study. Also, such close monitoring of behaviour would have introduced an observational bias. However, it is possible that the women who developed hyponatraemia had larger hourly oral fluid intake leading to symptomatic hyponatraemia. Tiredness and irritability, initial symptoms of hyponatraemia, may have prompted energy supplement by intravenous glucose infusion. This alterative interpretation strengthens the need of registering all oral fluid intakes during labour and remembering hyponatraemia as possible diagnosis. Not all consecutive women were included, omitting many women in advanced labour. The proportion of nulliparas is therefore large in our study population, probably increasing the incidence of hyponatraemia. Infants were examined and treated according to departmental routine; therefore, only clinically significant symptoms were investigated. The results regarding neonatal outcome should be interpreted with these limitations in mind.