A congenital disorder caused by a baby having an extra copy of chromosome 18 (three instead of the normal two). Characteristics of the disorder include a large number of different malformed organs and malformed physical features of the face and skeletal structure. In most cases, the child dies before it is born; 90% of babies born live die within a year of birth. Symptoms may be less severe when the trisomy occurs after fertilization during mitosis in the zygote (10% of cases), than when the trisomy occurs during the meiosis which produced the egg or sperm (90% of cases). For more information, visit http://www.icondata.com/health/pedbase/files/TRISOMY2.HTM.