Given that nucleosynthesis (BBN) provides the oldest available information we have on the evolution of the Universe, it is worthwhile exploring possible deviations from the standard Big Bang cosmology based on General Relativity in pre-BBN epochs. Non-GR pre-BBN cosmologies affect the evolution of the WIMP DM component, in particular determining relevant shifts on its freeze-out temperature. The impact of this new degree of freedom in the discussion of naturalness of WIMPs in playing the role of DM is presented in the context of Scalr Tensor (ST) theories of gravity.