Three are the essential parameters needed to describe seismicity: the b-value of the Gutenberg-Richter relation and the spatial and temporal fractal dimensions (Ds and Dt). Several cases have been reported when a significant decrease in the b-value or in the spatial fractal dimension preceded major earthquake sequences or aftershocks. Here we use the fractal method based on the correlation integral to study the temporal changes in the spatial (2-D) distribution of earthquakes in three important seismic zones of Italy. In all three zones Ds shows significant variability, which correlates well with the major events and clearly marks the beginning and the end of an earthquake cycle. Finally, the nucleation of most major events is associated with a fractal dimension value which corresponds to the topological dimension of a plane.