Down syndrome is the most common chromosome abnormality, it occurs in 1 in every 600 births and is caused by an extra copy of chromosome 21. The disorder is also known as trisomy 21.
Down syndrome symptoms include distinct physical features, intellectual disability, heart defects, digesting track defects, vision and hearing disorders and other health issues. Many people with Down syndrome cannot lead an independent, normal life.
Due to Down syndrome’s high prevalence, prenatal testing is offered to parents including screening tests for detecting the disorder. These tests are offered to all pregnant women according to age and risk factors.
Down syndrome risk factors
Down syndrome can occur in general population, but certain people have a greater chance of having a baby with this disorder. Some of the risk factors include:
Older maternal age
One of the most significant risk factors for chromosome abnormalities, and particularly Down syndrome, is maternal age. The older the mother, the chance of having a child with Down syndrome increases. This is because “older” eggs are at higher risk of aberrations during chromosome division, which may lead to chromosome abnormalities.
While a 20-year-old mother’s chance to have a baby with Down syndrome is 1 to 1,500, a 35-year-old mother’s chance is 1 to 350, and a 45-year-old mother’s probability is 1 to 35. Women over the age of 35 are recommended amniocentesis due to the higher risk for chromosome defects and other disorders.
Family history of another child with Down syndrome
Families with one child with Down syndrome are at higher risk of having another baby with the same disorder (around 1% chance).
Diagnosis before and during pregnancy
Due to the high prevalence of Down syndrome, parents are offered different genetic tests for detecting Down syndrome as part of prenatal screening tests. These tests include:
Genetic testing for parents
Couples with a risk for genetic defects (such as a family history of chromosome abnormalities) are recommended genetic counselling even before the pregnancy, to make sure they have no DNA abnormalities.
Nuchal translucency test and maternal blood screening
The first genetic test women undergo during the pregnancy is a nuchal translucency test that is preformed in weeks 11 – 13 of pregnancy. This is a simple ultrasound test that can accurately detect a risk for Down syndrome.
In addition, mothers are referred to maternal blood screening, also known as biochemical prenatal screening. Combined with the results of the nuchal translucency test, this blood test can provide information about the risk for Down syndrome in over 80% of cases.
Alpha-fetoprotein test
This is a blood test performed in the second trimester, in weeks 16-18 of pregnancy. The test detects a risk for Down syndrome, trisomy 18 and neural tube defects at around 65%. When combined with nuchal translucency and maternal blood screening results, alpha-fetoprotein tests can provide more accurate information on the chances for Down syndrome.
Chorionic villus sampling (CVS)
CVS Is performed in weeks 10 – 13 in pregnancies with a risk of over 1:200 for Down syndrome, according to first trimester blood screening and nuchal translucency results. The test uses a sample of cells from the placenta, this sample is sent to a lab for a chromosome analysis.
The test is performed in the early stages of pregnancy and allows parents to decide whether to continue the pregnancy before later stages when this procedure becomes more difficult.
Amniocentesis
Amniocentesis is performed in weeks 16-20 of pregnancy. It is recommended in pregnancies with a risk of 1:380 according to the alpha-fetoprotein test results, or according to genetic counselling after insufficient test results.
During the test, a small sample of amniotic fluid is sampled and sent to a lab. The test is performed at a later stage than CVS, and so allows deciding on terminating the pregnancy only at later, more advanced stages.
