Causes and risks Return to top Trisomy 18 is a relatively common syndrome affecting approximately 1 out of 8,000 live births and affecting girls more than twice as often as boys. Multiple abnormalities are associated with the presence of an extra number 18 chromosome , many which are not compatible with more than a few months of life.
Few infants survive beyond the first year.
There may be many other abnormalities noted. Prevention Return to top Intrauterine diagnosis of trisomy 18 is possible with an amniocentesis and chromosome studies on amniotic cells. Parents who have a child with translocational trisomy 18 should have chromosome studies, because they are at increased risk for another child with trisomy 18. At the birth of the child, an unusually small placenta may be noted. Chromosome studies show trisomy 18, partial trisomy, or translocation .