  We consider perturbative modifications of the Friedmann equations in terms of
energy density corresponding to modified theories of gravity proposed as an
alternative route to comply with the observed accelerated expansion of the
universe. Assuming that the present matter content of the universe is a
pressureless fluid, the possible singularities that may arise as the final
state of the universe are surveyed. It is shown that, at most, two coefficients
of the perturbative expansion of the Friedman equations are relevant for the
analysis. Some examples of application of the perturbative scheme are included.
