  Magnetization measurements as a function of temperature are reported for
$YBa_{2}Cu_{3}O_{6.5}$ crystal (T_{c}=45.2 K) for fields between 0.2 to $3.5\
Tesla$. All isochamps for H>1 Tesla intersect at $T_{2D}^{\star}\simeq 42.8\
K$, implying fluctuations contribution to the magnetization. These curves
collapse into a single curve when magnetization and temperature are scaled
according to the predicted ''2D scaling'' in the fluctuation regime.
Surprisingly, the low field curves also intersect, at $T_{3D}^{\star}\simeq
43.4 K$, and they obey a 3D scaling. We provide a theoretical picture of the
magnetization in the fluctuation regime based on the Lawrence-Doniach model.
Within this model we calculate the field and temperature dependence of the
magnetization. The two intersection points and the $2D\to 3D$ crossover are
consistent with the experimental observation.
