The specific heat of a single crystal of YBa2Cu3O7 was measured by adiabatic calorimetry between 40 and 150 K. The typical scatter of the data is well below 0.1%. Fields up to 20 T were applied either parallel or perpendicular to the c-axis. The shape of the anomaly at Tc changes with the magnetic field in the same manner for both directions, within a scale factor that determines the bulk anisotropy ratio, 5.5 ± 0.5, for (dHc2/dT)Tc. The superconducting transition is still observable in 20 T. A growing mixed-state Maki term is present. The characteristic non-classical behavior of YBa2Cu3O7 in a field, featuring a considerable smearing of the transition and an apparently field-independent onset, may be qualitatively understood as a phenomenological consequence of the small and anisotropic value of the coherence length ξ alone, using the relevant models of field-induced critical 1D fluctuations on the one hand, and a London-like regime for the mean-field part of the transition on the other hand.