During pregnancy, when is it a must to avoid laying on the right side? and can propping belly on a pillow be ok to lay on right side?

Mainly during labor: Avoiding the right side is mainly used during labor when there are fetal heart rate abnormalities as a tool to increase blood flow to the placenta. This labor maneuver has now been adopted as a recommendation throughout pregnancy without much scientific evidence to support it. You may adjust with pillows in whatever way is comfortable for you