The high-order harmonic generation process was studied using laser pump pulses of different duration, down to 7 fs. The time profile of this kind of pulse corresponds to a few optical cycles at the fundamental frequency, and therefore a cycle-to-cycle variation of the maximum amplitude occurs. These facts violate the adiabatic approximation of the generation process, giving rise to new aspects of the physical process. The comparison of results with pulses in the two regimes are reported and discussed. The modeling of the complete generation and propagation process in three dimensions is also presented.