  We derive a simple formula for the periods associated with the low energy
effective action of $N=2$ supersymmetric $SU(2)$ Yang-Mills theory with massive
$N_f\le 3$ hypermultiplets. This is given by evaluating explicitly the integral
associated to the elliptic curve using various identities of hypergeometric
functions. Following this formalism, we can calculate the prepotential with
massive hypermultiplets both in the weak coupling region and in the strong
coupling region. In particular, we show how the Higgs field and its dual field
are expressed as generalized hypergeometric functions when the theory has a
conformal point.
