Add to ORKGA measure for the twist of Gaussian light is expressed in terms of the second-order moments of the Wigner distribution function. The propagation law for these second-order moments between the input plane and the output plane of a first-order optical system is used to express the twist in one plane in terms of moments in the other plane. Although in general the twist in one plane is determined not only by the twist in the other plane, but also by other combinations of the moments, several special cases are considered for which a direct relationship between the twists can be formulated. In particular it is shown under what conditions zero twist is preserved in a first-order optical system.