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  • Kayla Hui

Down Syndrome Blood Test

Down syndrome is the most common chromosomal condition diagnosed in the United States. Down syndrome is a condition in which a person has an extra chromosome. This can negatively impact a host of different functions within the body, often leading to lower IQ and various health problems. For this reason, many parents who carry the genetic possibility of having a child with Down syndrome may want to be aware. Down syndrome screening tests can detect whether or not an unborn child is more likely to have Down syndrome. Other tests may confirm or rule out this diagnosis. These diagnostic tests during pregnancy can confirm or rule out a diagnosis but may have a small risk of causing a miscarriage.

For clarification, as previously stated Down syndrome is a condition where a person has an extra chromosome. Usually, chromosomes are paired together, this extra chromosome is known as a trisomy. This extra chromosome changes how the body and brain development resulting in mental and physical changes. Although people with Down syndrome may look similar or act similar, individuals with Down syndrome have different abilities from each other.

There are three types of Down syndrome, however, due to physical features and behaviors being similar it is difficult to tell without viewing their chromosomes. There is the most common type of Down syndrome, Trisomy 21, about 95% of people with Down syndrome have Trisomy 21. Trisomy 21 is when the individual's karyotype has 3 copies of chromosome 21. Translocation Down Syndrome occurs in a small percentage of people when an extra part or the entire chromosome 21 is “trans-located”, or attached, to a different chromosome rather than being a separate chromosome 21. Mosaic Down Syndrome refers to a mixture or combination of chromosomes. Children with mosaic Down syndrome may have some cells with 3 copies of chromosome 21 while others have only 2 copies. Although the individuals with mosaic Down syndrome have similar features to the other types of Down syndrome, they may have fewer features of the condition due to some of their cells having the typical number of chromosomes. Researchers are aware that Down syndrome is caused by an extra chromosome, but it is unclear what other factors can cause Down syndrome. As stated, there are tests to detect Down syndrome during pregnancy.

There are two types of tests available to detect Down syndrome during pregnancy: screening tests and diagnostic tests. A screening test can tell whether a pregnancy has a lower or higher chance of having Down syndrome, but does not provide an absolute diagnosis. However, screening tests are often safer for the mother and the developing child. Screening tests often include a combination of blood tests which help to screen the number of various substances in a mother’s blood and ultrasound.

The blood tests portion of a screening test measures the various quantities of alpha-fetoprotein, inhibin A, plasma protein A, estriol, and human chorionic gonadotropin. These are all markers for down syndrome. During the ultrasound, the technician views the fluid behind the fetus’ neck. Extra fluid behind the neck may indicate a genetic problem. The ultrasound will also check for physical features associated with Down syndrome in the fetus. Combined with the mother’s age these can estimate her chance of having a child with Down syndrome. The screening tests are often offered in the first and second trimesters and are only able to predict about 80% of fetuses with Down syndrome.

A more recent innovation is noninvasive prenatal screenings. These tests involve blood taken from the mother as early as 10 weeks of gestation. It uses the detection of cell-free DNA that circulates between the fetus and the expectant mother. Recent research has shown that this new blood test can detect up to 98.6% of fetuses with trisomy 21.

Diagnostic tests are performed after a positive screening test to confirm a Down syndrome diagnosis. This is done afterward as it is typically more invasive. There are multiple types of diagnostic tests. There is Chorionic villus sampling (CVS) which examines a sample of chorionic villi from the placenta for testing. The chorionic villi are projections in the placental tissue which share the baby’s genetic makeup. CVS is often performed much earlier than other prenatal diagnostic tests. Amniocentesis examines the amniotic fluid or the fluid from the sac surrounding the baby. The procedure removes the amniotic fluid from the uterus for testing or treatment. Amniotic fluid contains fetal cells and various proteins. Percutaneous umbilical blood sampling (PUBS) or Cordocentesis is a test that examines blood from the umbilical cord. However, the use of cordocentesis is becoming rarer due to the other diagnostic procedures having a lower risk of fetal death. These tests look for changes in the chromosomes that could indicate a Down syndrome diagnosis.

Due to the intense impacts Down syndrome holds families must be fully informed before proceeding with a pregnancy. This wide variety of tests allows mothers to make important decisions before giving birth to the child, especially whether or not they have the capabilities to care for a child with down syndrome. Parents have the option to keep the fetus to raise them or put them up for adoption. Many families are willing to adopt a child with Down syndrome. There are also options to terminate the fetus. This innovation holds great importance and will continue to impact and improve the lives of many.

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