Based in the field of breastfeeding, a breast pump apparatus (100) for expressing breast milk is disclosed. The breast pump apparatus (100) comprises a container unit (110) for receiving expressed breast milk, and a pump unit (120) including a pump body (130) and a pumping device (140) for applying negative pressure to a pump volume defined by the pump body (130). The pump body (130) comprises an inlet portion (132) adapted to be received on a female breast and an outlet portion (134) adapted to channel expressed milk to the container unit (110). The pump body (130) comprises an actuator element (200, 300, 400) comprising an active material capable of deforming upon application of a control signal to it, wherein deformation of the actuator element modifies at least one of a shape and size of the pump volume.