Many changes in the respiratory system occur during pregnancy, particularly during the third trimester, which can alter respiratory function during sleep, increasing the incidence and severity of sleep disordered breathing. These changes include increased ventilatory drive and metabolic rate, reduced functional residual capacity and residual volume, increased alveolar-arterial oxygen gradients, and changes in upper airway patency. The clinical importance of these changes is indicated by the increased incidence of snoring during pregnancy, which is likely also to reflect an increased incidence of obstructive sleep apnoea/hypopnoea syndrome. For the respiratory physician asked to review a pregnant patient, the possibility of sleep disordered breathing should always be considered. This review first examines the normal physiological changes of pregnancy and their relationship to sleep disordered breathing, and then summarises the current knowledge of sleep disordered breathing in pregnancy.