We study the ground states of a planar quantum dot with N=5,6,7 electrons, in the presence of a perpendicular magnetic field. Using a spatially unrestricted Hartree Fock technique followed by spin and angular momentum symmetry restoration, chemical potentials are calculated and transitions between different ground states are identified. A spin blockade in the 6–>7 transition is found. The structure of the quantum dot wave functions is illustrated by their electron densities.