The present invention relates to a method of sensing shape of an optical fiber sensor having an optical fiber, the optical fiber having embedded therein a plurality of fiber cores including a central fiber core extending along a longitudinal center axis of the optical fiber and at least one outer fiber core arranged in a radial distance from the longitudinal center axis and helically wound around the longitudinal center axis with a spin rate. The method comprises acquiring, from an optical distributed strain measurement along the plurality of fiber cores, a bend phase signal indicative of bend strain along the optical fiber sensor and a twist phase signal indicative of twist strain along the optical fiber sensor, and reconstructing the shape of the optical fiber sensor from the bend phase signal and the twist phase signal by relating the bend phase signal to curvature along the optical fiber sensor using a first scaling factor fbend and by relating the twist phase signal to a twist rate using a second scaling factor ftwist. At least one of the first scaling factor fbend and the second scaling factor ftwist is taken as being dependent on position along the optical fiber sensor, so as to take into account of at least one of: a varying radial distance of at least one of the plurality of fiber cores from the longitudinal center axis along the optical fiber sensor and a varying spin rate of the at least one outer fiber core along the optical fiber sensor. Further, an optical shape sensing system is described.
