The mean-field electronic specific heat C-e of a superconductor in the presence of a magnetic field is first calculated. The model contains the following as ingredients: an energy spectrum presenting saddle points in the band structure, a Josephson coupling between CuO planes and a d(x2-y2) gap symmetry. Superconductivity fluctuations are next extracted from published data on the specific heat of a YBa2Cu3O7-x and of an HgBa2Ca2Cu3O8 single crystal. Critical XY and Gaussian regimes are deduced from mean-field calculations. Magnetic field cross-overs are evaluated and physical parameters such as the Ginzburg-Landau number and the distance between CuO planes are calculated as tests.