A chromosomal or mendelian aetiology

A chromosomal or mendelian aetiology has been identified
for many multiple congenital malformation syndromes
enabling appropriate recurrence risks to be given. When the
aetiology of a recognised multiple malformation syndrome is
not known, empirical figures for the risk of recurrence derived
from family studies can be used, and these are usually fairly
low. The genetic abnormality underlying de Lange syndrome,
for example, is not yet known, but recurrence risk is very low.
Consanguineous marriages may give rise to autosomal recessive
syndromes unique to a particular family. In this situation, the
recurrence risk for an undiagnosed multiple malformation
syndrome is likely to be high. In any family with more than one
child affected, it is appropriate to explain the 1 in 4 risk of
recurrence associated with autosomal recessive inheritance,
although some cases may be due to a cryptic familial
chromosomal rearrangement.