Fetal mortality in oral cleft families (VI): a search for early embryonic and zygotic mortality.

A hypothesis concerning the detection of early embryonic and zygotic mortality in oral cleft families is presented. It is suggested that probands from solitary pregnancies are the result of a higher liability to clefting which eliminated potential siblings prior to the conception of the proband. A positive association between the degree of liability to clefting and fetal mortality has already been established by the authors' previous work. In sibships of solitary probands, such liability is assumed to have caused early embryonic and zygotic mortality which eliminated potential siblings and recognizable fetal loss. This hypothesis can be tested by an examination of the sex-ratios in probands from solitary pregnancies and those from multiple pregnancies. For CL(P), a decrease in the proportion of males would be expected in probands from solitary pregnancies in comparison to those from multiple pregnancies. For CP, however, an increase in the proportion of males in solitary probands is expected in comparison to those from multiple pregnancies. This hypothesis is applied to 613 CL(P) probands and 317 CP probands from Indiana. Although the expected decrease (for CL(P)) and increase (for CP) in the proportion of males in solitary probands is not quite significant, the trends are in the expected directions. It is suggested that the hypothesis should be tested further with additional data on oral clefts and other similar anomalies.