Increased circulating progesterone associated with pregnancy relaxes smooth muscle and causes relaxation of the lower oesophageal sphincter, whereas placental gastrin increases the volume and decreases the pH of gastric contents. The enlarging uterus increases intragastric pressure and there is an increase in small and large bowel transit time. However, evidence suggests that gastric emptying per se is not affected by pregnancy though it may be decreased in labour if opioids are given.
Extradural analgesia with local anaesthetic solutions in labour is associated with normal gastric emptying, whereas subarachnoid or extradural opioids (fentanyl or diamorphine) in large doses cause a modest decrease in gastric emptying. Systemic opioid analgesia causes a much greater and prolonged decrease in gastric emptying. However, recent randomised studies have demonstrated large gastric volumes and a high incidence of vomiting in women allowed to eat solid food, even when pain was adequately controlled with a low-dose fentanyl/bupivacaine epidural.
Plasma progesterone concentrations return to non-pregnant values within 24 hours of delivery, and gastroesophageal reflux is considerably reduced within 48 hours of delivery. The period of risk of aspiration thus extends to an ill-defined time after delivery, and appropriate general anaesthetic management in the early postpartum period is thus somewhat controversial.
Routine withholding of food and fluids in labour has been challenged by a number of authors, particularly those who are not anaesthetists. They point out that absolute starvation is not popular with mothers, that aspiration associated with emergency general anaesthesia nowadays is uncommon and that there may be risks associated with prolonged starvation. On the other hand, there is little evidence that a period of starvation during labour is harmful, although it may be unpleasant. Starvation is associated with ketosis, but this has not been found to affect the duration or outcome of labour.
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