In this paper, we report Cooper Pairs Distribution function D_cp(ω, T_c) for bcc Niobium under pressure. This function reveals information about the superconductor state through the determination of the spectral regions for Cooper-pairs formation. D_cp(ω, T_c) is built from the well-established Eliashberg spectral function and phonon density of states, calculated by first-principles. D_cp(ω, T_c) for Nb suggests that the low-frequency vibration region (ω<6meV) is where Cooper-pairs are possible. From D_cp(ω, T_c) , it is possible to obtain the N_cp parameter, which is proportional to the total number of Cooper-Pairs formed at a temperature T_c. The N_cp parameter allows an approach to the understanding of the Nb T_c anomalies, measured around 5 and 50 GPa.