Article Sections

Introduction

Trisomy 18 (Edwards syndrome) is a chromosomal disorder characterized by the presence of 3 copies of chromosome 18.  It is the second most common autosomal trisomy (after trisomy 21) and is associated with severe congenital anomalies, severe intellectual disability, and high neonatal mortality.

Pathogenesis and risk factors

Normally, homologous chromosomes separate during meiosis, resulting in 4 gametes with 1 copy of each chromosome.  However, in meiotic nondisjunction, a pair of homologous chromosomes fail to separate, resulting in 4 gametes with 0, 1, or 2 copies of the chromosome in each gamete ( Figure 1).  If the gamete with 2 copies of the chromosome is fertilized by a gamete with 1 copy of the chromosome, then the zygote has 3 copies of the chromosome.  Maternal meiotic nondisjunction

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