Sleep in pregnancy is essential for maternal and fetal health, especially during the third trimester. It is well understood that supine sleep in late pregnancy reduces maternal-fetal blood flow as the weight of the gravid uterus may cause compression of the inferior vena cava  and/ or supine hypertension . Maintaining a left lateral tilt while lying down/ sleeping relieves inferior vena caval compression by the gravid uterus and is thus recommended to pregnant women. Asking women to sleep on their left side is the simplest and cheapest intervention but that is easier said than done.
Pregnant women spend approximately 130 hours sleeping in a supine position in their last trimester, averaging 5.8 hours of supine sleep per night . To determine the feasibility of asking pregnant women to maintain a left lateral tilt when sleeping, researchers J Dorrian and J Warland designed a study in which all participants (n=30 pregnant women in their third trimester, with singleton, uncomplicated pregnancies) were asked to settle to sleep on their left side and settle back to sleep on their left side if they wake up in the middle of the night .
Participants were asked to record their sleeping position, waking position and re-settling position in a sleep diary provided to them, along with an estimate of how long they think they slept on their left side. Participants were recorded with a night-capable infrared camera as they slept and their diary entries were compared to their video data to determine the accuracy of self-reported sleep position in late pregnancy. The results showed that participants were able to sleep on their left side for an average of 60% of the night (ranging from 11% to 98% across participants) and that self-reported and video determined left-side sleep time had a moderate correlation (r=0.48, mean difference=3 mins, SD = 3.5h), but there were large individual differences in reporting accuracy. This study was able to show that it is feasible to ask pregnant women to increase the time they spend sleeping on their left side.
That being said, it should be noted that sleeping on the left side may result in reduced sleep duration for pregnant women who usually do not sleep on that side. A similar study done by McIntyre JP, Ingham CM, Hutchinson BL, et al. found that pregnant women accurately recalled their sleep onset position, sleep duration and the number of times they got out of bed however sleep latency and sleep position on waking were not accurately recalled. They also suggested that pregnant women should settle into bed on their left side as the initial going-to-sleep position is maintained the longest throughout the night .