  With the goal of determining the $\theta_{13}$ neutrino oscillation mixing
angle, the measurements of reactor antineutrino fluxes at the Double Chooz,
RENO and Daya Bay experimental facilities have uncovered a systematic
discrepancy between the number of observed events and theoretical expectations.
In the \emph{ab initio} approach, the total reactor antineutrino spectrum is a
weighted sum of spectra resulting from all $\beta$ branches of all fission
products in the reactor core. At all three facilities a systematic deviation of
the number of observed events from the number of predicted events was noticed,
i.e., approximately 6\% of the predicted neutrinos were not observed. This
discrepancy was named the reactor neutrino anomaly. In theoretical studies it
is assumed that all the decays are allowed in shape, but a quarter of all
transitions are actually forbidden and may have a complex energy dependence
that will affect the total reactor antineutrino spectrum. In order to estimate
the effect of forbidden transitions, we perform a fully self-consistent
calculation of spectra from all contributing transitions and compare the
results with a purely allowed approximation.
