A drive mechanism for generating a negative pressure in a vacuum chamber (32/132) of a breast pump (10/110), the drive mechanism comprising a housing (12/112), a motor (14/114) coupled to a rotatable drive element within the housing, a resilient membrane (28/128) coupled to the housing and, a flexible strap (50/150) coupled between the drive element and the resilient membrane. Rotation of the drive element causes the flexible strap to pull and resiliently deform the resilient membrane to create the negative pressure in a vacuum chamber of a breast pump. Also, a breast pump including such a drive mechanism together with a vacuum chamber (32/132) coupled to the housing (12/112). The vacuum chamber comprises a receptacle and a breast-receiving funnel (40/140) in fluid communication with the receptacle, wherein when a woman's breast is placed in the breast-receiving funnel, the vacuum chamber is sealed closed such that deformation the resilient membrane (28/128) creates a negative pressure in the vacuum chamber.