A progressing cavity pump adapted for pumping of compressible fluids, comprising an inner rotor (1) having a number of thread-starts (Z) together with an adapted stator or outer rotor (2) provided with one extra thread-start (Z+l), wherein a number of, in principle closed pump cavities (6) are formed which are moved, during fluid conveyance, from the inlet side (A) of the pump to the outlet side (B) of the pump, at which position they become open outlet cavities (6c) exposed to the fluid pressure in a downstream pipeline, and wherein at least one passage is disposed between the outlet side (B) and the, in principle, closed pump cavity (6b) defined closest to the outlet side (B), wherein said passage is structured for intentional fluid back-flow from the outlet side (B) in a measured and approximately constant volume.