(also known as trisomy
18 or trisomy E) is a congenital
disorder caused by a baby
having an extra copy of chromosome
18 (in other words, the baby has three copies instead of the normal
Characteristics of the disorder include widespread defects in internal organs throughout the body and malformed physical features of the face and skeletal structure. Specific facial deformities include an abnormally small skull (microcephaly), an abnormally small lower jaw, malformed ears, and a webbed neck.
In most cases, the child dies before it is born; 90% of babies born alive die within a year of birth. Symptoms from the disorder may be less severe when the trisomy occurs after fertilization during mitosis in the zygote than when the trisomy occurs during the meiosis which produced the egg or sperm.