- intellectual disability- flat facies- prominent epicanthal folds- single palmar crease- gap between 1st 2 toes- duodenal atresia- Hirschsprung disease- congenital heart disease (eg, atrioventricular septal defect)- Brushfield spots - early-onset Alzheimer disease- ↑ risk of ALL and AML

- 95% due to meiotic nondisjunction- 4% of cases due to unbalanced Robertsonian translocation (chromosomes 14 and 21)- Only 1% of cases due to postfertilization mitotic error

- Incidence 1:700

- First trimester ultrasound: ↑ nuchal translucency and hypoplastic nasal bone- ↑ β-hCG, ↑ inhibin A- ↓ AFP, PAPP-A, estriol