This chromosome disorder is caused by a foetus having an extra copy of chromosome 18. Although it occurs in only 1 in 8000 births, it is probably the most common chromosome defect at the time of conception.
There is a high caesarean rate of undiagnosed cases. Following birth, half of the infants die by the age of 5 days and 90% die by the age of 100 days. Maternal serum screening using the Triple Test, can detect 60% of cases, with only nine cases classified as abnormal.