  We present results on the nucleon axial form factors within lattice QCD using
two flavors of degenerate twisted mass fermions. Volume effects are examined
using simulations at two volumes of spatial length $L=2.1$ fm and $L=2.8$ fm.
Cut-off effects are investigated using three different values of the lattice
spacings, namely $a=0.089$ fm, $a=0.070$ fm and $a=0.056$ fm. The nucleon axial
charge is obtained in the continuum limit and chirally extrapolated to the
physical pion mass enabling comparison with experiment.
