A syndrome characterized by the presence of an extra (third) chromosome on an otherwise diploid chromosome 18 associated with a broad spectrum of variable abnormalities consisting of more than 130 individual defects of the craniofacial structures, brain, heart, kidneys, and gut. One-third of newborn infants (weighing about 2300) are premature and two-thirds are female. Fetal abnormalities consist mainly of polyhydramnios, small placenta, and single umbilical artery. Tumors in some cases. Trisomy 18 mosaicism is often associated with normal intelligence and mild phenotype.