Fluids during labour
Practice and policies regarding oral intake during labour show large variations throughout the world. A liberal attitude seems to prevail in the UK, many European countries, and Australia, and some countries even allow solids during labour.9 Clear oral fluids are recommended during uncomplicated labour in the USA.24‘Nil per mouth’ policies are often challenged, particularly by midwives, and create the need of intravenous fluid administration. Maternal starvation may cause fetal acidosis, and several studies address the safety of glucose administration.25,26 Solutions containing glucose are usually hypotonic despite electrolyte addition, although near isotonic solutions have been used for study purposes.26
Two studies indicate that women receiving 250 ml per hour of Ringer's lactated solution intravenously had shorter duration of labour and less need for oxytocin than those receiving 125 ml per hour.7,8 The assumption regarding beneficial influence of larger volumes of fluids during labour is largely based on literature in the field of sports medicine.7,8,27 For optimal muscle performance, athletes are recommended to drink ‘the maximal amount tolerated’ as the sensation of thirst is believed to underestimate the real fluid requirements during exercise.27 Thirst is, however, a strong physiologic stimulus, effectively protecting against dehydration when fluids are readily available. No physiologic warning system protects the body against over-hydration. Therefore, suppression of thirst by abundant drinking implies exposure to the risk of hyponatraemia. Since 1991, several deaths due to hyponatraemic encephalopathy have occurred during endurance competitions. Common recognition of excessive drinking as the main cause of these disasters was, however, delayed until 1995.28,29
The duration of labour equals a marathon for many women, hence the importance of careful administration of fluids. Many authors consider 150–200 ml per hour safe to drink during labour, but with simultaneous intravenous administration, this could well be an excessive amount as illustrated by the present study.1,12,13 Also, the tonicity of fluids determines their potential for causing hyponatraemia, and clear oral fluids are invariably hypotonic. Sport drinks are somewhat confusingly described as isotonic. However, their osmolality, even when similar to that of plasma, is largely made up of carbohydrates. Their content in sodium is less than half compared with plasma, rendering these drinks hypotonic. Some prospective randomised trials have been designed to study the impact of sport drinks and carbohydrate intake on labour duration and outcome.30–33 These studies show conflicting results, but all have in common the lack of control of electrolyte status in the study participants.