Trisomy 18 is a severe hereditary disease caused by an additional chromosome 18. Affected children have various malformations, disturbed growth, delayed development, and reduced intelligence. In the first 5 years of life, 90% of those affected die because of heart or kidney anomalies, eating disorders, infections, or apnea. Children with mosaicism or partial trisomy 18 have a better prognosis. Hepatoblastomas and Wilms tumors are the most common tumors. The risk of developing a Wilms tumor is around 1%.