We consider instabilities of a single mode with finite wavenumber in inversion symmetric spatially one-dimensional systems, where the character of the bifurcation changes from sub- to supercritical behaviour. Starting from a general equation of motion the full amplitude equation is derived systematically and formulae for the dependence of the coefficients on the system parameters are obtained. We emphasize the importance of nonlinear derivative terms in the amplitude equation for the behaviour in the vicinity of the bifurcation point. In particular the numerical values of the corresponding coefficients determine the region of coexistence between the stable trivial solution and stable spatially periodic patterns. Our approach clearly shows that similar considerations fail for the case of oscillatory instabilities.