Other autosomal trisomies

Trisomy 18 (Edwards syndrome)
Trisomy 18 has an overall incidence of around 1 in 6000 live
births. As with Down syndrome most cases are due to
nondisjunction and the incidence increases with maternal age.
The majority of trisomy 18 conceptions are lost spontaneously
with only about 2.5% surviving to term. Many cases are now
detectable by prenatal ultasound scanning because of a
combination of intrauterine growth retardation,
oligohydramnios or polyhydramnios and major malformations
that indicate the need for amniocentesis. About one third of
cases detected during the second trimester might survive to
term. The main features of trisomy 18 include growth
deficiency, characteristic facial appearance, clenched hands
with overlapping digits, rocker bottom feet, cardiac defects,
renal abnormalities, exomphalos, myelomeningocele,
oesophageal atresia and radial defects. Ninety percent of
affected infants die before the age of 6 months but 5% survive
beyond the first year of life. All survivors have severe mental
and physical disability. The risk of recurrence for any trisomy is
probably about 1% above the population age-related risk.
Recurrence risk is higher in cases due to a translocation where
one of the parents is a carrier.