A diagnosis can often be made before birth by testing for low levels of alpha-fetoprotein in the mother's blood. If low levels are detected, a sample of the amniotic fluid will confirm the diagnosis. After the birth, the infant's blood will be sampled to confirm the presence of trisomy 21. An increased risk of leukemia and heart disease is present in children with DS that will reduce the chance of survival until adulthood. Hearing loss may occur due to chronic ear infections and the associated accumulation of inner ear fluid.
Vision loss is also common due to deterioration of the cornea and lens.
Adults with DS may begin to develop symptoms of dementia in their 30s such as memory loss, which further affects intelligence and personality. DS patients display motor and kinetic deficit. Their physical appearance is usually limp, with stooping shoulders, dropped lower jaw with mouth open, and a gawky and straddling gait. These conditions can be greatly improved when diligent physiotherapy is initiated in early infancy and continued through later years. It was considered hopelessly untreatable, a belief that dominated the scientific community and produced no new research on DS for more than 100 years.