SufentanilFentanylMorphinansDiprenorphineReceptors, OpioidReceptors, Opioid, muAnalgesia, ObstetricalAnalgesics, OpioidPositron-Emission TomographyAnalgesia, EpiduralBupivacaineBrainAlfentanilAnesthetics, IntravenousInjections, SpinalAnestheticsAnesthesia, IntravenousAnesthesia, SpinalAnesthetics, CombinedAnesthetics, LocalLabor, ObstetricPreanesthetic MedicationPain, PostoperativeAdjuvants, AnesthesiaLabor Stage, FirstDouble-Blind MethodAnesthesia, ObstetricalAnalgesia, Patient-ControlledSubarachnoid SpaceAnalgesiaAmidesPruritusAnesthesia Recovery Period