The physiology of maternal sleep (and sleep position) in healthy late pregnancy

Abstract

Unexplained stillbirth is the leading cause of antenatal death in developed countries, accounting for 71- 103 unborn children dying each year in New Zealand between 2007 and 2013. Recent evidence suggests maternal sleep and sleep position may be associated with an increased risk of late stillbirth. If something as simple as changing maternal sleep position, or even reducing the time spent in a certain position, could decrease stillbirth risk, then it clearly warrants further investigation. This thesis aimed to describe sleep in healthy late pregnancy, particularly as it relates to maternal sleep position. It was hypothesised that changes in maternal sleep position would produce measurable physiological effects. This thesis found that conventional measures of maternal physiology when awake, and conventional measures of sleep-disordered breathing when asleep, were unable to detect an effect of changing maternal position. However, using more detailed analyses of data collected during a respiratory sleep study demonstrated that supine sleep produces pronounced physiological effects. The right-lateral position demonstrated a physiological effect when awake, consistent with previous case-control studies demonstrating increased risk of the right-lateral sleep position, but no physiological difference was demonstrated here during sleep. This suggests that avoiding the supine sleep position is important, but right-lateral is no more harmful than left-lateral. The physiological and behavioural descriptions of sleep demonstrated how healthy late pregnancy differs from healthy non-pregnant women, and thus how analyses can be modified to detect important effects in this population. Whilst potentially-harmful physiological effects of the supine position were demonstrated, the pregnant women in this thesis display physiological and behavioural characteristics that appear to protect against prolonged exposure. Finally, this thesis demonstrated that mothers in late pregnancy can recall sleep with moderate accuracy, particularly sleep-onset position, which was implicated in stillbirth studies. This thesis reports a number of physiological and behavioural observations that support the findings in previous New Zealand stillbirth case control studies, and described normal physiology and behaviour in healthy late pregnancy. It has also developed assessments of maternal physiology during sleep that can be replicated in future studies of healthy and high-risk pregnancies, with recommendations on how future research can expand on the methodologies used here.