An electric heating pad for warming a patient includes a heated underbody support or heated mattress. The heated underbody support or mattress includes a heater assembly having a flexible sheet-like heating element, conductive bus bars attached at or near side edges of the heating element, and fabric side edge extensions attached to heating element side edges. The heated underbody support or mattress also includes a layer of polymeric foam positioned under the heater assembly and a shell, including at least two sheets of flexible material, covering at least a portion of the heater assembly and the layer of polymeric foam. When the heater assembly is wrapped around the top surface of the layer of polymeric foam, side edges of the heating element extend partially down the two side walls of the layer of polymeric foam and the conductive bus bars lie adjacent those two side walls.