We show that if $v$ is a regular semi-classical form (linear functional), then the symmetric form $u$ defined by the relation $x\sigma u=-\lambda v$, where $\sigma u$ is the even part of $u$, is also regular and semi-classical form for every complex $\lambda$, except for a discrete set of numbers depending on $v$. We give explicitly the recurrence coefficients, integral representation and the structure relation coefficients of the orthogonal polynomials sequence associated with $u$ and the class of the form $u$ knowing that of $v$. We conclude with some illustrative examples.