Earth is flattened at the poles and bulges at the equator. This is due to the fact that

Earth is bulged out at equator and flattered at poles because of centripetal force. The earth spins at constant rate but rate of movement is different the equator is moving fastest and poles are not moving (ignoring the fact that earth is orbiting the sun). Because of this movement centrifugal force is pulling matter closer to equator which structure outwards giving earth slightly non-spherical shape.