The clinical features and morphological findings in 31 Japanese infants with 
trisomy 18 are presented. The majority were small-for-date infants. There was no 
sex predominance in our series, as opposed to male:female ratios of 1:3 reported 
in the literature. The average age at death was greater in females than in 
males. Cardiovascular anomalies were consistently present; ventricular septar 
defect and patent ductus arteriosus being the most common malformations. Various 
other internal malformations including the Arnold-Chiari malformation were 
observed.