Down Syndrome

Dive Deeper

Sometimes children can have a missing or extra chromosome, a condition known as aneuploidy.  Down syndrome is a disorder caused by the presence of an extra copy of chromosome 21, also referred to as trisomy 21.  This genetic anomaly occurs at conception, when the man’s sperm fuses with a woman’s egg to form a single-cell embryo.  Most children with Down syndrome survive to birth, often with medical conditions, such as congenital heart defects, eye disease, thyroid disease, and hearing loss.  With appropriate medical care, children born with Down syndrome can lead healthy, happy lives with an average life expectancy of 60 years.1

With appropriate medical care, children born with Down syndrome can lead healthy, happy lives. (Image Credit: Science Source)

Down syndrome can be diagnosed before or after birth.  Standard prenatal screening for Down syndrome is often performed during the first and second trimester to calculate the risk of having a baby with trisomy 21. Maternal age, blood tests examining biochemical markers, and fetal nuchal translucency measurements taken during an ultrasound are traditional ways to screen for Down Syndrome early in pregnancy.2

Traditional screening for trisomy 21 may be combined with other DNA screening and diagnostic testing, usually between 10-18 weeks gestation, to increase the chance of correctly predicting whether a child has Down syndrome. Diagnostic DNA tests can be performed using fetal samples obtained via amniocentesis and chorionic villus sampling.  These tests are more accurate than traditional screening measures, but involve taking samples from the amniotic sac or placenta thereby increasing the risk for pregnancy loss.3

A new, advanced method of non-invasive prenatal screening (NIPS) uses cell-free fetal DNA found in the mother’s blood to screen for trisomy 21.  Scientists can detect cell-free fetal DNA from a mother’s blood sample as early as 4 weeks and 5 days after fertilization. However, testing is usually done after 8 weeks.4

This is the karyotype for a human female with Down's Syndrome. It is characterized by an extra copy of genetic material on chromosome 21. (Image Credit: Science Source)

All screening tests, including the NIPS screen, have a high false-positive rate.  This means that a test result may say the baby has Down syndrome, when in fact the child does not have Down syndrome.5