Theoretical recurrence risks for cleft lip derived from a population of consecutive newborns.

Theoretical recurrence risks for cleft lip with or without cleft palate (CL(P)) were calculated from heritability estimates derived from a population of 203 newborns with CL(P) in a total of 220,927 consecutive births in north-east Italy. Birth prevalence of CL(P) and the frequency of CL(P) in relatives of probands were estimated after exclusion of cases with CL(P) resulting from a known cause or pathogenesis. The method allowed estimation of the theoretical recurrence risk for any family by considering the total number of affected and unaffected first, second, and third degree relatives. The lower value of the theoretical risk compared to the empirical risk, obtained from retrospective data of selected families, was the result of methodological differences.