[Why the incidence of congenital and hereditary disorders usually is zero]

Incidence is a frequency measure for quantifying new cases of a disease, and prevalence quantifies patients who already have a particular disorder. An exception to this rule is the frequency of congenital and hereditary disorders. If this frequency is expressed as the proportion of the number of births, prevalence is appropriate. Incidence is a meaningful measure only when used to indicate the frequency with which such a disorder is diagnosed during life, as opposed to the frequency with which it originates.