An alarm (35) provided by a fall detection system (2) may be caused by an accidental drop of the system. Therefore prior to issuing the alarm the fall detection system a confirmation is needed that a potential fall originates from a fall detection system that is worn by a user (4). A fall of a fall detection system that is not attached to a user is characterized by the occurrence of one or more full rotations of the system. Said rotations are identified by analyzing the output signal of a magnetometer, and by detecting a periodicity in said output signal. The fall detection system (2) provides the alarm in dependence of an identified absence of at least one full rotation.