The present invention relates to a breast pump (1) for extracting milk from a female breast comprising an expression kit (2), a container (5) connected to the expression kit (2), and a vacuum unit (6) connected to the expression kit (2), wherein the combined volumes of the expression kit (2), the container (5), the vacuum unit (6) and the connections thereof define a system air volume, and a vacuum pressure sensor for determining volume changes in the system air volume in response to changes in vacuum pressure.
