How do they diagnose downs?

Exam and blood test. Initially the diagnosis of down syndrome is based on the baby's appearance. If the newborn displays some or all of the characteristics of down syndrome, the doctor will order a test called a chromosomal karyotype. This blood test evaluates the child's chromosomes. If there's an extra chromosome 21 present in all or some of the cells, the diagnosis is down syndrome, or trisomy 21.