A patient support apparatus comprises a substantially rigid support deck (2) for supporting a patient support element, a first sensing element (9) and a second sensing element (9) arranged on the support deck (2) and separated from one another in a direction along the width of the patient support apparatus. Each sensing element is configured to determine, in use, the force or pressure applied to it by the patient support element and a patient on the patient support element. One or more processors (10, 11, 12), coupled to the sensing elements, are configured to determine the location of the centre of gravity of a patient on the patient support element, along the width of the patient support apparatus, based upon outputs of the first and second sensing elements. An alarm (17) may be activated when the location of the centre of gravity along the width of the patient support apparatus is outside a predetermined range of values.
