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.