Fetal mortality in oral cleft families (IX): factors relating to the occurrence of sporadic clefts.

Pedigree data on 854 probands with cleft lip and/or cleft palate from the State of Indiana are presented. These include 123 probands with cleft lip alone (CL), 453 with cleft lip and palate (CLP), and 278 with isolated cleft palate (CP). Probands and families were interviewed at the Indiana University Medical Center during the years 1962-1980. Among features of special interest are an excess of bilateral CLP (46% of all CLP cases) and a significantly greater number of older mothers and fathers (over 34 years old) than in the general population. An apparent increase in the proportion of sporadic cases with time may be due to bias in ascertainment. It is suggested that the increased proportions of more severe clefts and older parents are among several factors which contribute to the incidence of fetal mortality in a cleft population. The association of fetal mortality with liability to clefting introduces a more precise way to define a sporadic cleft. That is, a truly sporadic cleft is one conceived as a single occurrence event and has survived to become a liveborn cleft child whereas other apparently sporadic cleft individuals represent the surviving cleft in a sibship in which fetal mortality has eliminated all other cleft offspring already conceived. This observation has applications to the collection and interpretation of genetic pedigree data for clefts as well as many other genetic conditions.