How is Down syndrome diagnosed?
Your doctor may suggest that you have tests during pregnancy to find out if your baby has Down syndrome. A screening test can help show if the baby (fetus) is at risk for Down syndrome. But these tests sometimes give false-positive results, meaning they show a problem when there is none. Screening tests include: • A blood test and a fetal ultrasound late in the first trimester. The ultrasound looks for thickness in the back of the baby’s neck (nuchal fold), which can be a sign of Down syndrome. This kind of ultrasound is not available in many areas. • A blood test called a maternal serum triple screen or quadruple screen in the second trimester. By measuring substances in your blood, these tests can estimate the chance that your baby has Down syndrome or other health problems. Your doctor may suggest a chromosome (karyotype) test if you have an abnormal screening test result or if you are worried about Down syndrome. A karyotype test has more risks than a screening test, but it can say
Expectant mothers can choose to have a screening or diagnostic test performed during pregnancy to help determine the risk of having a baby with the Down syndrome, or whether or not the child will actually have the condition. Down syndrome can also be identified at birth or shortly thereafter, based on distinctive physical characteristics common in babies with the condition. More often a chromosome test, called a karyotype, is administered after birth to confirm the diagnosis.
Down syndrome can be diagnosed in infancy based on the characteristic clinical findings. When Down syndrome is suspected in a person, a genetic test called a chromosome analysis is performed on a blood or skin sample to look for an extra chromosome 21 (trisomy 21). Trisomy 21 means that each cell in the body has three copies of chromosome 21 instead of the usual two copies. Having an extra number 21 chromosome interrupts the normal course of development, causing the characteristic clinical features of Down syndrome. Some people who have Down syndrome have an extra number 21 chromosome in only some of their body’s cells. This type of Down syndrome is called mosaic Down syndrome. A small number of individuals have Down syndrome because part of chromosome 21 becomes attached (translocated) to another chromosome before or at the time of conception. These individuals have two copies of chromosome 21, and additional material from chromosome 21 that is attached to another chromosome. The chro
