Mandarin, or Sweet Orange, Neroli, and a hint of Nutmeg has been an effective diffuser combo. I always suggest to diffuse no longer than 5 minutes out of the half hourt. Aromatherapy will be only as effective as the emotional state behind it's use. No matter what oils are choosen. The earlier that positive associations and imagergy is started the more ingrained it will be for the birth.
Nothing has more influence over our physical functioning than our beliefs and assumptions.
Surrendering to the birthing process is also important. In reality there is no failure in birth. Realizing this up front can save alot of disappointments. A healthy baby is the only true goal. This happens, and needs to be honored.