  At astrophysical and cosmological scales, there is a detectable amount of
magnetic field. There are several probable origins for this observed magnetic
field, including the possibility of its origin in the early Universe. There are
several models for primordial magnetogenesis, and if the inflationary
background is taken into account, broken conformal invariance is required to
generate a sufficient amount of magnetic field. The breaking of conformal
invariance is introduced either by new couplings between electromagnetic field
and inflaton field or including higher derivative terms to the theory. As a
step to unify these different approaches in the literature, we propose an
Effective Field Theory (EFT) approach based on expansion about the Hubble
parameter $(H)$ and its derivatives, where EFT parameters describe the
magnetogenesis scenario in the early Universe, and different choices of
parameters correspond to different models. We explicitly show that the
generation of primordial magnetic fields requires two necessary conditions --
conformal invariance breaking and causal propagation. While broken conformal
invariance is a common requirement for primordial magnetogenesis, for the first
time, we show that causal propagation is also a necessary condition. We confirm
this by considering a specific model of primordial magnetogenesis.
