This syndrome is caused by the presence of an extra
chromosome 18 - i.e. there are three number 18 chromosomes instead of the
normal two. The syndrome results in death in 90% of infants either during pregnancy
or before they are a year old. The estimated frequency of live births is 1:5000,
and it increases with the mother's age. It is four times more common in females
than in males.
Fetuses with this syndrome present with a wide spectrum of findings, of which the
main ones are
intrauterine growth retardation, a small head, typically clenched hands, heart defects, kidney defects, etc. Up to
now, approximately 130 defects associated with trisomy 18 have been described. Fetuses
with this trisomy in only some of their cells (mosaics) have a better chance of
survival.
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