Prenatal Genetic Screening Tests – ACOG

Posted: March 17, 2018 at 3:42 pm

Pregnancy

Prenatal genetic testing gives parents-to-be information about whether their fetus has certain genetic disorders.

Genetic disorders are caused by changes in a persons genes or chromosomes. Aneuploidy is a condition in which there are missing or extra chromosomes. In a trisomy, there is an extra chromosome. In a monosomy, a chromosome is missing. Inherited disorders are caused by changes in genes called mutations. Inherited disorders include sickle cell disease,cystic fibrosis, TaySachs disease, and many others. In most cases, both parents must carry the same gene to have an affected child.

There are two general types of prenatal tests for genetic disorders:

Both screening and diagnostic testing are offered to all pregnant women.

Screening tests can tell you your risk of having a baby with certain disorders. They include carrier screening and prenatal genetic screening tests:

First-trimester screening includes a test of the pregnant womans blood and an ultrasound exam. Both tests usually are performed together and are done between 10 weeks and 13 weeks of pregnancy:

Second-trimester screening includes the following tests:

The results from first- and second-trimester tests can be combined in various ways. Combined test results are more accurate than a single test result. If you choose combined screening, keep in mind that final results often are not available until the second trimester.

Cell-free DNA is the small amount of DNA that is released from the placenta into a pregnant womans bloodstream. The cell-free DNA in a sample of a womans blood can be screened for Down syndrome, trisomy 13, trisomy 18, and problems with the number of sex chromosomes. This test can be done starting at 10 weeks of pregnancy. It takes about 1 week to get the results. A positive cell-free DNA test result should be followed by a diagnostic test with amniocentesis or CVS.

The cell-free DNA screening test works best for women who already have an increased risk of having a baby with a chromosome disorder. For a woman at low risk of having a baby with a chromosome disorder, conventional screening remains the most appropriate choice. Cell-free DNA testing is not recommended for a woman carrying more than one fetus.

Results of blood screening tests for aneuploidy are reported as the level of risk that the disorder might be present:

Diagnostic testing with CVS or amniocentesis that gives a more definite result is an option for all pregnant women. Your obstetrician or other health care professional, such as a genetic counselor, will discuss what your screening test results mean and help you decide the next steps.

With any type of testing, there is a possibility of false-positive results and false-negative results. A screening test result that shows there is a problem when one does not exist is called a false-positive result. A screening test result that shows there is not a problem when one does exist is called a false-negative result. Your health care professional can give you information about the rates of false-positive and false-negative results for each test.

It is your choice whether to have prenatal testing. Your personal beliefs and values are important factors in the decision about prenatal testing.

It can be helpful to think about how you would use the results of prenatal screening tests in your pregnancy care. Remember that a positive screening test tells you only that you are at higher risk of having a baby with Down syndrome or another aneuploidy. A diagnostic test should be done if you want to know a more certain result. Some parents want to know beforehand that their baby will be born with a genetic disorder. This knowledge gives parents time to learn about the disorder and plan for the medical care that the child may need. Some parents may decide to end the pregnancy in certain situations.

Other parents do not want to know this information before the child is born. In this case, you may decide not to have follow-up diagnostic testing if a screening test result is positive. Or you may decide not to have any testing at all. There is no right or wrong answer.

Amniocentesis: A procedure in which a needle is used to withdraw and test a small amount of amniotic fluid and cells from the sac surrounding the fetus.

Aneuploidy: Having an abnormal number of chromosomes.

Carrier Screening: A test done on a person without signs or symptoms to find out whether he or she carries a gene for a genetic disorder.

Cell: The smallest unit of a structure in the body; the building blocks for all parts of the body.

Chorionic Villus Sampling (CVS): A procedure in which a small sample of cells is taken from the placenta and tested.

Chromosomes: Structures that are located inside each cell in the body and contain the genes that determine a persons physical makeup.

Cystic Fibrosis: An inherited disorder that causes problems in digestion and breathing.

Diagnostic Tests: Tests that look for a disease or cause of a disease.

DNA: The genetic material that is passed down from parents to offspring. DNA is packaged in structures called chromosomes.

Down Syndrome: A genetic disorder that causes abnormal features of the face and body, medical problems such as heart defects, and intellectual disability. Most cases of Down syndrome are caused by an extra chromosome 21 (trisomy 21). Many children with Down syndrome live to adulthood.

Fetus: The stage of prenatal development that starts 8 weeks after fertilization and lasts until the end of pregnancy.

Genes: Segments of DNA that contain instructions for the development of a persons physical traits and control of the processes in the body. It is the basic unit of heredity and can be passed down from parent to offspring.

Genetic Counselor: A health care professional with special training in genetics and counseling who can provide expert advice about genetic disorders and prenatal testing.

Genetic Disorders: Disorders caused by a change in genes or chromosomes.

Inherited Disorders: Disorders caused by a change in a gene that can be passed down from parent to children.

Monosomy: A condition in which there is a missing chromosome.

Mutations: Permanent changes in genes that can be passed on from parent to child.

Neural Tube Defects: Birth defects that result from incomplete development of the brain, spinal cord, or their coverings.

Nuchal Translucency Screening: A test in which the size of a collection of fluid at the back of the fetal neck is measured by ultrasound to screen for certain birth defects, such as Down syndrome, trisomy 18, or heart defects.

Obstetrician: A physician who specializes in caring for women during pregnancy, labor, and the postpartum period.

Placenta: Tissue that provides nourishment to and takes waste away from the fetus.

Screening Tests: Tests that look for possible signs of disease in people who do not have symptoms.

Sex Chromosomes: The chromosomes that determine a persons sex. In humans, there are two sex chromosomes, X and Y. Females have two X chromosomes and males have an X and a Y chromosome.

Sickle Cell Disease: An inherited disorder in which red blood cells have a crescent shape, causing chronic anemia and episodes of pain. It occurs most often in African Americans.

TaySachs Disease: An inherited birth defect that causes intellectual disability, blindness, seizures, and death, usually by age 5 years. It most commonly affects people of Eastern and Central European Jewish, Cajun, and French Canadian descent, but it can occur in anyone.

Trimester: One of the three 3-month periods into which pregnancy is divided.

Trisomy: A condition in which there is an extra chromosome.

Trisomy 13 (Patau Syndrome): A chromosomal disorder that causes serious problems with the brain and heart as well as extra fingers and toes, cleft palate and lip, and other defects. Most infants with trisomy 13 die within the first year of life.

Trisomy 18 (Edwards Syndrome): A chromosomal disorder that causes severe intellectual disability and serious physical problems such as a small head, heart defects, and deafness. Most of those affected with trisomy 18 die before birth or within the first month of life.

Ultrasound Exams: Tests in which sound waves are used to examine internal structures. During pregnancy, they can be used to examine the fetus.

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Prenatal Genetic Screening Tests – ACOG

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