We report on isofield curves of ͱ −M vs T, where M is the reversible magnetization, of YBa 2 Cu 3 O 6.95 , YBa 2 Cu 3 O 6.65 , and Bi 2 Sr 2 CaCu 2 O 8+x with the magnetic field, H, applied parallel to the c axis of the samples ͑and also parallel to the ab planes for YBaCuO͒. For temperatures close to the critical temperature, T c , the quantity ͱ −M is proportional to the order parameter amplitude ͉͉. Curves of ͱ −M vs T allowed to study the asymptotic behavior of the form ͑T a − T͒ m of ͉͉ near T c , as a function of field. Results for the studied samples produced values of T a ͑H͒ lying above T c , suggesting that the magnetic field gradually allows to probe a region of temperatures where phase correlations persist above T c . The study performed here in YBaCuO samples allowed to study how phase correlations evolve with doping in the pseudogap region of YBaCuO. ͱ −M vs T curves for all samples show a rather large amplitude fluctuation with no phase correlation extending well above T a ͑H͒ which is interpreted in terms of a Gaussian Ginzburg-Landau approach with a total-energy cutoff in the fluctuation spectrum. Resulting values for the exponent m found for all samples, 0.5Ͻ m Ͻ 0.7, are interpreted as due to phase fluctuations of the d-wave pairing symmetry of the order parameter in the ab planes.