A breast pump assembly (13) comprising:-a non-removably mounted optical sensor (3, 6, 7, 9, 10, 11, 12) configured for measuring an optical characteristic of milk that has just been expressed from a breast and has entered the breast pump (13)；-a control unit configured for:-comparing an output signal of the optical sensor (3, 6, 7, 9, 10, 11, 12), representing said measured optical characteristic, with data representing a corresponding optical characteristic of a sample of milk having a known fat content to determine the fat content of said expressed milk； and-determining an indication of the amount of milk remaining in the breast from the determined fat content of said expressed milk.
