Edwards syndrome (also known as trisomy 18 [T18]) is a chromosomal abnormality caused by the presence of all, or part of, an extra 18th chromosome. This genetic condition almost always results from nondisjunction during meiosis. It is named after John Hilton Edwards, who first described the syndrome in 1960. It is the second-most common autosomal trisomy, after Down syndrome, that carries to term.