One of the most difficult poses for people drawing the figure, is the reclining or lying down pose. One of the reasons is we are unfamiliar with the human figure in this pose, so tends to throw our perception of the figure as we are more used to seeing the human form stranding, or sitting. The biggest problem with this pose is the amount of foreshortening!
This is where measuring, using the thumb & pencil method is best and will deal with the problem, leading to a successful finished piece.