We consider the problem of ultraviolet divergences in the decoherence facto
r in quantum cosmology. It is shown that using dimensional regularization i
t is not possible to obtained the consistent reduced density matrix. In the
case of tracing out the bosonic inhomogeneous degrees of freedom one can g
et a reasonable reduced density matrix showing decoherence properties using
a conformal reparametrization of fields, which suppresses ultraviolet dive
rgences. In the case of fermionic fields such a suppression can be achieved
by performing of a non-local Bogoliubov transformation of Grassmann variab
les.