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Understanding the Results of Prenatal Genetic Testing

By on July 3, 2018

The results of prenatal testing may not always be straightforward and may require some interpretation. Additionally, making decisions regarding prenatal testing requires that you understand the results. Prenatal testing for genetic disorders is comprised of two types of tests: prenatal screening tests and prenatal diagnostic tests. 

Prenatal Screening Tests

Prenatal screening tests are blood tests offered to every woman, regardless of age or health. These tests evaluate the likelihood that the fetus may havea genetic disorder, such as a chromosomal abnormality or an inherited genetic disorder. Prenatal screenings are used to measure the amount of certain substances, such as hormones and proteins, in the mother’s blood. For example, low levels of a substance called alpha-fetoprotein may indicate Down syndrome.

Additionally, noninvasive prenatal genetic testing is used to analyze cells from the placenta that contain DNA from the fetus. Using a blood sample from the mother, these cells are isolated and analyzed in order to understand the risk that the child might have a chromosomal abnormality. A positive result from a noninvasive prenatal testing will indicate that the baby has a high risk of being born with a chromosomal disorder.

Ultrasound imaging is sometimes used in conjunction with blood tests. Ultrasound is used to monitor the integrity of the baby’s organs, as some genetic disorders are characterized by severe abnormalities in certain organs. For example, a structural heart defect may indicate Down syndrome or another type of genetic disorder.


If the results of any prenatal screening test suggest that there is a high risk that the fetus has a genetic disorder, your genetic counselor will explain to you exactly what these results mean. They will also present you with the option of doing further testing, such as prenatal genetic diagnostic testing.

Prenatal Diagnostic Tests

Prenatal genetic diagnostic testing is only offered to women who have received a positive or abnormal result from a screening test. Prenatal diagnostic tests tell your doctor definitivelyif your baby has a genetic disorder. Prenatal diagnostic tests use samples from the placenta or the amniotic fluid that contain DNA from the fetus.

Prenatal diagnostic tests include:

Amniocentesis – Uses a sample of the amniotic fluid

Chorionic villus sampling (CVS) – Uses a sample of cells from the placenta

A negative result from a prenatal diagnostic test means that the child is normal and that there is no genetic condition detected. However, if the child was only tested for one disorder, a negative result will indicate that the child does not have the one specific condition. There may still be a possibility that the child has a different genetic condition that was not tested for.

A positive test result means that the child has been diagnosed with a certain genetic disorder. If you receive a positive result from a prenatal genetic diagnostic test, your genetic counselor will explain to you what this means. They will also discuss if treatment is available as well as answer any questions you have regarding the condition.

Work With a Genetic Counselor

If you are considering prenatal testing, it is recommended that you work with a genetic counselor. A genetic counselor is a healthcare professional that is specially trained in medical genetics and counseling. These individuals help expecting couples understand the risk that they may have a child with a chromosomal abnormality or an inherited genetic disorder. If couples choose to undergo prenatal testing, genetic counselors will discuss what the results of prenatal screening and prenatal diagnostic tests mean as well as give couples information on possible next steps.










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