  We uncover a new pathway towards multiferroicity, showing how magnetism can
drive ferroelectricity without relying on inversion symmetry breaking of the
magnetic ordering. Our free-energy analysis demonstrates that any commensurate
spin-density-wave ordering with a phase dislocation, even if it is collinear,
gives rise to an electric polarization. Due to the dislocation the electronic
and magnetic and inversion centers do not coincide, which turns out to be a
sufficient condition for multiferroic coupling. The novel mechanism explains
the formation of multiferroic phases at the magnetic commensurability
transitions, such as the ones observed in YMn$_{2}$O$_{5}$ and related
compounds. We predict that in these multiferroics an oscillating electrical
polarization is concomitant with the uniform polarization. On the basis of our
theory we put forward new types of magnetic materials that are potentially
ferroelectric.
