Fetal mortality in sibships of cases with neural tube defects.

It has been suggested that rates of fetal mortality in sibships of probands with a malformation inherited as a multifactorial threshold trait may reflect their liability to the malformation. If so, spontaneous abortion rates should be more frequent in sibships thought to have greater liability. For anencephaly and spina bifida (ASB), then, spontaneous abortion should be higher in the sibships of male probands and in families with more than one affected case (multiplex families). This hypothesis was tested using data on cases from The Montréal Children's Hospital and from the literature. Approximately 5000 pregnancies were analyzed. Rates of abortion did not vary with the sex or diagnosis of the proband. The spontaneous abortion rate was slightly higher in multiplex than in simplex families, but the difference was not statistically significant and most likely reflects the differing reproductive patterns in the two types of families. Thus, if male probands and multiplex sibships do have, on average, more liability for ASB, this liability cannot be detected in spontaneous abortion rates in the sibships available for this analysis.