How Prenatal and Postnatal Genetic Testing Works Video

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  • 0:55 Why Genetic Testing is…
  • 1:38 Prenatal Genetic Testing
  • 5:10 Postnatal Genetic Testing
  • 7:41 Lesson Summary
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Instructor: Sarah Lawson

Sarah has taught nursing courses and has a master's degree in nursing education.

Have you ever wondered why pregnant women have so many doctor visits and need so many tests done? Learn more about how prenatal and postnatal genetic testing works and test your knowledge with a quiz.

Why Genetic Testing is Important

Genetics play a really important role in our health. Just about every human disease has a genetic component. Today we know about more than 6,000 known genetic disorders that affect an estimated 12 million Americans. Because genes are so influential, it is a good idea to get to know our genes so that we can better understand how they might impact our health. One of the ways that we can learn about genes is through genetic testing.

A genetic test is the analysis of human deoxyribonucleic acid (DNA), ribonucleic acid (RNA), chromosomes, and proteins. Genetic testing, also known as genetic screening, is used to detect disease-related genotypes, mutations, phenotypes or karyotypes that can be passed from parents to offspring. Genetic tests examine DNA at the chromosomal level to detect additional, absent or mutated chromosomal material that can cause inherited diseases. These tests are important for the purposes of diagnosis, treatment, and other clinical decision making.

In this lesson, we'll explore two types of genetic testing: prenatal and postnatal genetic tests.

Prenatal Genetic Testing

Prenatal genetic testing is testing performed during the pregnancy to help doctors to diagnose diseases or disease potential in the fetus. Most prenatal genetic tests are completed by drawing a blood sample from the mother. For example, the triple marker screen is a blood test that measures levels of alpha fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol.

AFP is made by the fetal yolk sac and fetal liver; a small amount crosses the placenta and is found in the mother's blood. Elevated levels of AFP are associated with conditions such as spina bifida, and low levels are associated with Down syndrome. Spina bifida occurs when the bones of the spine (vertebrae) don't form properly. Down syndrome can cause physical growth delays and intellectual disability.

HCG is a glyco-protein produced by the placenta. It is normally elevated at the time of implantation and drops at approximately eight weeks; it drops again around 12 weeks' gestation. Elevated levels of hCG are associated with Down syndrome. The placenta also produces unconjugated estriol. Lower levels of unconjugated estriol are commonly associated with Down syndrome.

Women who have abnormal results with the triple marker screen are often advised to have additional testing, such as chorionic villus sampling, amniocentesis or percutaneous umbilical blood sampling, which requires withdrawal of a blood sample from the umbilical cord.

Chorionic villus sampling (CVS) is the genetic testing of cells that will become the placenta. This test is performed during the first three months of the pregnancy using a small, flexible catheter to extract the cells that will form into placental tissue (chorionic villi tissue). These cells are genetically identical to the baby's cells and contain the same DNA and chromosomes. The cells are examined for genetic disorders such as cystic fibrosis (CF), Down syndrome, Tay-Sachs, and Thalassemia. Cystic fibrosis is a disease of the mucus and sweat glands. Tay-Sachs is a nervous system disease, and Thalassemia is a blood disorder.

Amniocentesis is a genetic test that involves the use of a needle to extract amniotic fluid from the uterus. This test is usually performed at 15 to 20 weeks' gestation but it can be performed earlier. Fetal karyotyping, DNA analysis, and biochemical testing, can be performed on the cells obtained from amniocentesis. These tests will allow doctors and the baby's parents to know of chromosomal abnormalities and the gender of the baby.

Postnatal Genetic Testing

Postnatal genetic testing is testing performed on the baby after delivery to allow doctors to diagnose diseases. This testing is important because each year in the U.S., 1 in 33 babies will be born with a chromosomal abnormality. Before being discharged from the hospital, newborns undergo genetic screening. This screening, called a newborn screen, is a blood sample from the baby that is used to screen for specific genetic abnormalities. For example, this screening tests for phenylketonuria (PKU), congenital hypothyroidism and other medical conditions that are genetically inherited.

PKU is the result of a deficiency of an enzyme called phenylalanine hydroxylase. This enzyme is responsible for breaking down protein in our food. PKU can cause intellectual disability, organ damage, and postural problems. Children born with PKU can live normal, healthy lives but need to pay close attention to their dietary intake of protein.

Congenital hypothyroidism is a condition that results from a partial or complete loss of thyroid function. Conditions like PKU and hypothyroidism are best discovered early, which is why testing is performed on all babies before discharge from the hospital.

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