SufentanilAnalgesics, OpioidReceptors, Opioid, muReceptors, OpioidFentanylReceptors, Opioid, deltaReceptors, Opioid, kappaAnalgesia, ObstetricalOpioid PeptidesAnalgesia, EpiduralBupivacaineAlfentanilInjections, SpinalAnesthetics, IntravenousAnestheticsNaloxoneNarcotic AntagonistsAnesthesia, IntravenousMorphineAnesthesia, SpinalAnesthetics, LocalAnesthetics, CombinedPain, PostoperativePreanesthetic MedicationLabor, ObstetricOpioid-Related DisordersNarcoticsAdjuvants, AnesthesiaDouble-Blind MethodNaltrexoneAnalgesiaLabor Stage, FirstPain MeasurementAnalgesia, Patient-ControlledEnkephalinsAnesthesia, ObstetricalEnkephalin, Ala(2)-MePhe(4)-Gly(5)-AnalgesicsPruritusSubarachnoid SpaceDrug ToleranceEnkephalin, D-Penicillamine (2,5)-AmidesAnesthesia Recovery PeriodEndorphinsPainDynorphinsMidazolamAnesthesia, EpiduralInjections, EpiduralPropofolBuprenorphine