FAQ Pregnancy
FAQ165, April 2014



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Screening Tests for Birth Defects

What is a birth defect?

A birth defect is a problem that is present at birth, although it may not be noticed until the child is older. Birth defects may affect any part of the body, including major organs such as the heart, lungs, or brain. The defect may affect the baby’s appearance, a body function, or both.

What causes birth defects?

Some birth defects are caused by problems with chromosomes. Others are caused by a gene that is passed from parent to child. Some birth defects result from exposure to harmful agents.

What are some examples of chromosome disorders?

Aneuploidy is a condition in which there are missing or extra chromosomes. The most common aneuploidy is called a trisomy, in which there is an extra chromosome. A common trisomy is trisomy 21 (Down syndrome). Other trisomies include trisomy 13 (Patau syndrome) and trisomy 18 (Edwards syndrome).

A monosomy is a condition in which there is a missing chromosome. A common monosomy is Turner syndrome, in which a female has a missing or damaged X chromosome.

What are inherited disorders?

Inherited disorders are caused by defective genes. These disorders are passed down by parents to their children. Some inherited disorders are more common in certain races and ethnic groups, such as sickle cell disease (African American), cystic fibrosis (non-Hispanic white), and Tay–Sachs disease (Ashkenazi Jewish, Cajun, and French Canadian).

What other things can cause birth defects?

Birth defects also may be caused by exposure to harmful agents, such as medications, chemicals, and infections. Some birth defects may be caused by a combination of factors. For most birth defects, the cause is not known.

How can I find out if I am at increased risk of passing on a genetic disorder?

Your health care provider or a genetic counselor can help find out if you are at increased risk of passing on a genetic disorder by asking about your personal and family health history.

What factors may increase my risk of passing on a genetic disorder?

Most babies with birth defects are born to couples without risk factors. However, the risk of birth defects is higher when certain factors are present. You are at increased risk if

  • you have a genetic disorder
  • you already have a child who has a genetic disorder
  • there is a family history of a genetic disorder
  • you belong to an ethnic group that has a high risk of certain genetic disorders

What types of prenatal tests are available to address concerns about birth defects?

The following prenatal tests are available:

  • Carrier tests—These screening tests can show if a person carries a gene for an inherited disorder. Carrier tests can be done before or during pregnancy. Cystic fibrosis carrier screening is offered to all women of reproductive age because it is one of the most common genetic disorders.
  • Screening tests—These tests assess the risk that a baby will have Down syndrome and other chromosome problems, as well as neural tube defects. These tests do not tell whether the fetus actually has these disorders.
  • Diagnostic tests—These tests can provide information about whether the fetus has a genetic condition and are done on cells obtained through amniocentesis, chorionic villus sampling, or, rarely, fetal blood sampling. The cells can be analyzed using different techniques.

What are the different types of screening tests for birth defects that can be performed during pregnancy?

Screening tests are performed during different trimesters of pregnancy. The following table lists the different types of screening tests:

Table 1. Prenatal Screening Tests
Screening Test Test Type What Does It Screen for? Down Syndrome Detection Rate
Combined first trimester screening Blood test for PAPP-A and hCG, plus an ultrasound exam Down syndrome
Trisomy 13
Trisomy 18
82–87%
Second trimester single screen for neural tube defects Blood test for AFP Neural tube defects 85%
Second trimester triple screen Blood test for AFP, hCG, and estriol Down syndrome
Trisomy 18
Neural tube defects
69%
Second trimester quad screen Blood test for AFP, hCG, estriol, and inhibin-A Down syndrome
Trisomy 18
Neural tube defects
81%
Integrated screening Blood test for PAPP-A and an ultrasound exam in the first trimester, followed by quad screen in the second trimester Down syndrome
Trisomy 18
Neural tube defects
94–96%
Integrated screening
(blood test only)
Same as integrated screening but no ultrasound exam Down syndrome
Trisomy 18
Neural tube defects
85–88%
Contingent sequential

First trimester combined screening result:

  • Positive: diagnostic test offered
  • Negative: no further testing
  • Intermediate: second trimester screening test offered
Down syndrome
Trisomy 18
Neural tube defects
88–94%
Stepwise sequential

First trimester combined screening result:

  • Positive: diagnostic test offered
  • Negative: second-trimester screening test offered
Down syndrome
Trisomy 18
Neural tube defects
95%
Abbreviations: AFP, alpha-fetoprotein; hCG, human chorionic gonadotropin; PAPP-A, pregnancy-associated plasma protein A

Do I have a choice between having screening tests or having diagnostic tests?

If a screening test shows an increased risk of a birth defect, diagnostic tests may be done to determine if a specific birth defect is present. Diagnostic testing may be done instead of screening if a couple is at increased risk of certain birth defects. Diagnostic testing also is offered as a first choice to all pregnant women, even those who do not have risk factors. Your health care provider will discuss all of the testing options with you and recommend the tests that best fit your needs.

What are the advantages and disadvantages of diagnostic tests compared with screening tests?

The main benefit of having diagnostic testing instead of screening is that it tells you whether or not the baby will be born with a chromosome disorder or a specific inherited disorder. The main disadvantage is that diagnostic tests can pose some risks to the pregnancy.

Do I have to have these tests?

Although screening tests for birth defects are offered to all pregnant women, it is your choice whether to have them done. Knowing whether your baby is at risk of or has a birth defect beforehand allows you to prepare for having a child with a particular disorder and to organize the medical care that your child may need. You also may have the option of not continuing the pregnancy.

Glossary

Alpha-fetoprotein (AFP): A protein produced by a growing fetus; it is present in amniotic fluid and, in smaller amounts, in the mother’s blood.

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: A person who shows no signs of a particular disorder but could pass the gene on to his or her children.

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

Chorionic Villus Sampling: 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 person’s 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 in people who are believed to have or who have an increased risk of a disease.

Estriol: A substance made by the placenta and the liver of the fetus.

Fetus: The developing organism in the uterus from the ninth week of pregnancy until the end of pregnancy.

Gene: A segment of DNA that contains instructions for the development of a physical trait or control of a process in the body. Genes are the basic units 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.

Human Chorionic Gonadotropin (hCG): A hormone produced during pregnancy; its detection is the basis for most pregnancy tests.

Inhibin-A: A substance made by the placenta during pregnancy.

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

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

Pregnancy-Associated Plasma Protein-A (PAPP-A): A protein made by the fetus and placenta during pregnancy.

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

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.

Tay-Sachs Disease: An inherited birth defect that causes mental retardation, blindness, seizures, and death, usually by age 5 years. It occurs mostly in people of Eastern European Jewish (Ashkenazi Jews), Cajun, and French Canadian descent.

Trimesters: 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 genetic disorder that causes serious heart defects and other problems with development. Most infants with trisomy 13 die within the first year of life.

Trisomy 18 (Edwards Syndrome): A genetic disorder that causes serious mental and developmental problems. Most infants with trisomy 18 die within the first year of life.

Trisomy 21 (Down Syndrome): A genetic disorder in which abnormal features of the face and body, medical problems such as heart defects, and intellectual disability occur.

Turner Syndrome: A condition affecting females in which there is a missing or damaged X chromosome. It causes a webbed neck, short height, and heart problems.

Ultrasound Exam: A test in which sound waves are used to examine internal structures. During pregnancy, it can be used to examine the fetus.

If you have further questions, contact your obstetrician–gynecologist.

FAQ165: Designed as an aid to patients, this document sets forth current information and opinions related to women’s health. The information does not dictate an exclusive course of treatment or procedure to be followed and should not be construed as excluding other acceptable methods of practice. Variations, taking into account the needs of the individual patient, resources, and limitations unique to the institution or type of practice, may be appropriate.

Copyright April 2014 by the American College of Obstetricians and Gynecologists