  For the past decade observations of the alignement of PNe symmetries with
respect to the galactic disk have led to conflicting results. Recently
observational evidence for alignment between PNe and local interstellar
magnetic fields in the central part of the Galaxy ($b < 5^\circ$) has been
found. We studied the role of the interstellar magnetic field on the dynamical
evolution of a PN by means of an analytical model and from 3D MHD numerical
simulations. We test under what conditions typical ejecta would have their
dynamics severely modified by an interstellar magnetic field. We found that
uniform fields of $> 100\mu$G are required in order to be dynamically dominant.
This is found to occur only at later evolutionary stages, therefore being
unable to change the general morphology of the nebula. However, the symmetry
axis of bipolar and elliptical nebulae end up aligned to the external field.
This result can explain why different samples of PNe result in different
conclusions regarding the alignment of PNe. Objects located at high galactic
latitudes, or at large radii, should present no preferential alignment with
respect to the galactic plane. PNe located at the galactic centre and low
latitudes would, on the other hand, be preferentiably aligned to the disk.
Finally, we present synthetic polarization maps of the nebulae to show that the
polarization vectors, as well as the field lines at the expanding shell, are
not uniform even in the strongly magnetized case, indicating that polarization
maps of nebulae are not adequate in probing the orientation, or intensity, of
the dominant external field.
