Dear Kate
I’m not an expert on this aspect but did notice in one of the research papers referred to earlier in this thread that lying someone on their left side gave the greatest cardiac output (i.e. most oxygenated blood into the circulation) which implies that elevating the right side is preferable. My personal advice to women after 30 weeks is to lie on either side (left side from 34 weeks for OFP) but not to panic if they wake up having rolled over onto their back in their sleep. Remember that the mattress will be softer than a floor that women would lie on in a yoga class so the effect on the circulation would be less. I feel the the woman's body would tell her even in her sleep to roll on her side if supine was creating a circulation problem. A tip for a woman who is uncomfortable on her side is to suggest she tries something soft under the bottom sheet e.g. a single duvet to relieve the pressure on the hip.
Hope that's helpful
Marion