Many of the births I have been at the woman has at some point lain on her back for rest, but mostly she instinctively moves onto her left side or on all four for actual pushing. Myself I have birth twice in a semi recumbant position (on my back, sitting up slightly). Once was in the bath and once on dry land. As long as you are not lying flat there is no real reason why this would make birth more difficult, although some women with posterior babies find lying completely flat helps the baby turn into a better position for birth and instinctively adopt this position during labour.