In a breast pump, milk expressed from a user's nipple may be drawn into the pumping mechanism used to create a vacuum. This can lead to unhygienic situations because pumping mechanisms are difficult or impossible to clean by a user. The present invention relates to a breast pump comprising first and second pressure chambers (22, 23) and an actuator (20) disposed to fluidly divide the two chambers (22, 23). The actuator (20) is movable to generate an inverse pressure differential in the second chamber (23) when a pressure differential is generated in the first pressure chamber (22), or a pressure differential is released from the first pressure chamber (22). Furthermore, the actuator (20) comprises a first piston element (30) and a second piston element (32), and wherein the first pressure chamber (22) is disposed between the first piston element (30) and the second piston element (32).