Principal findings and interpretation
Hyponatraemia defined as plasma sodium ≤130 mmol/l was found in 21 women after delivery. The reduction in plasma osmolality confirms the dilutional origin of hyponatraemia. Hyperglycaemia might have contributed to hyponatraemia in the six hyperglycaemic women.15 All study participants drank similar amounts per hour and received moderate hourly volumes of intravenous fluids with a mean sodium content of 69 mmol/l. Intravenous fluids were thus hypotonic, but with a sodium content higher than the most common glucose solutions. Two-thirds of all fluids were orally ingested, and these fluids are invariably hypotonic. In women with longer duration of labour, the cumulative effect of approximately 300 ml of hourly fluid intake resulted in a significant reduction of plasma sodium levels. The duration of labour itself cannot explain the development of hyponatraemia. On the contrary, longer lasting labour without administration of fluids would result in dehydration and hypernatraemia. Hyponatraemia may possibly have a negative influence on the process of labour, as hyponatraemia was significantly correlated with longer duration of second stage, instrumental delivery, and emergency caesarean for failure to progress. In women delivered by emergency caesarean section, hyponatraemia was not caused by intraoperative fluids as this was present before initiating anaesthesia. Earlier studies analysed cord sodium concentrations and found no difference in maternal and cord sodium concentration.1,3 We analysed both umbilical arterial and venous sodium concentrations and found that umbilical arterial concentrations were higher than maternal levels but significantly correlated, indicating fetal equilibration with maternal hyponatraemia. In infants, large weight loss, hyperbilirubinaemia, and respiratory distress have been believed to be caused by fluid overload.4,5,12 In our study, weight loss in excess of 10% of birthweight was most frequent in fluid group 3 (P = 0.05), indicating a possible relationship with maternal fluid overload. The few cases of hyperbilirubinaemia and respiratory problems were evenly distributed, with no differences between the groups (Table 2).