Let $\pi \colon \mathcal {X}\to B$
be a family whose general fibre $X_b$
is a $(d_1,\,\ldots,\,d_a)$
-polarization on a general abelian variety, where $1\leq d_i\leq 2$
, $i=1,\,\ldots,\,a$
and $a\geq 4$
. We show that the fibres are in the same birational class if all the $(m,\,0)$
-forms on $X_b$
are liftable to $(m,\,0)$
-forms on $\mathcal {X}$
, where $m=1$
and $m=a-1$
. Actually, we show a general criteria to establish whether the fibres of certain families belong to the same birational class.