A one-way approximation is investigated for the computation of wave propagation in varying cross-section waveguides, under the assumption of weak variations. Using a change of variable on the transverse space co-ordinate, a modified parabolic equation is proposed, that takes into account as additional terms the effects of the cross-sectional change along the waveguide. Providing that the boundary condition at the non flat wall is properly implemented and by use of a high order complex Padé expansion, the proposed equation can be integrated with usual numerical schemes and leads to accurate results in a large variety of problems. The error on the transmitted field is then estimated according to the maximum slope of the waveguide, that proves relevant to estimate the range of validity of the method.