  We consider two-nucleon scattering close to threshold. Partial-wave
amplitudes are obtained by an analytic extrapolation of subthreshold reaction
amplitudes calculated in a relativistic formulation of chiral perturbation
theory. The constraints set by unitarity are used in order to stabilize the
extrapolation. Neutron-proton phase shifts are analyzed up to laboratory
energies $T_{{\rm lab}}\simeq250$ MeV based on the
next-to-next-to-next-to-leading order expression for the subthreshold
amplitudes. We find a reasonably accurate description of the empirical S- and
P-waves and a good convergence of our approach. These results support the
assumption that the subthreshold nucleon-nucleon scattering amplitude may be
computed perturbatively by means of the chiral expansion. The intricate soft
scales that govern the low-energy nucleon-nucleon scattering are generated
dynamically via a controlled analytic continuation.
