Objective: To discuss methodological factors that account for the wide variation in the reported prevalence rates of anomalies associated with oral clefts.
Conclusions: The published prevalences of associated anomalies vary considerably because of the following: (1) differences in case definition and inclusion/exclusion criteria; (2) length of time after birth that cases are examined; (3) variability of clinical expression of associated anomalies; (4) knowledge and technology available to produce syndrome delineation; (5) selection of patients, sources of ascertainment, and sample size; and (6) true population differences and changes in frequency over time.