The following information may help to address your question:
mutation of an family member must be identified before prenatal testing can be performed.
Requests for prenatal testing for conditions such as hypokalemic periodic paralysis that do not affect intellect and have some treatment available are not common. Although most centers would consider decisions about prenatal testing to be the choice of the parents, discussion of these issues is appropriate.
The appropriate time for testing your child is a decision which should be made with your doctor or a genetics professional. Genetics professionals are a source of information for individuals and families regarding genetic diagnosis, natural history, treatment, mode of inheritance, and genetic risks to other family members.