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Screening for Birth Defects

Pregnant women who have a history of medical conditions, previous pregnancies with medical issues, a family or ethnic background that is prone to certain genetic conditions, or whose previous exams indicate causes for concern are usually given screening tests for birth defects. These tests tell the obstetrician about the possible risk for giving birth to a child with birth defects or genetic conditions. 

How often do birth defects occur?

The majority of babies born in the United States are healthy. Only 2-3 infants out of every 100 newborns have major birth defects. (Birth defects, congenital defects, or malformations are medical issues that the child has from birth.) The cause for these defects remains a mystery in approximately 70 percent of the cases. In the other 30 percent of cases, the birth defect is the result of genes and chromosomes the baby inherited, or the mother's exposure to harmful medications or substances.

A birth defect can interfere with the way the body appears, functions, or both. Most birth defects do not seriously affect the baby's quality of life, but some can be very serious.

The majority of birth defects develop during the first trimester of pregnancy. It is possible for the obstetrician to detect some birth defects before the baby is born with special screening tests. Other birth defects only appear at birth or later on in life. The birth defects that can be detected through screening tests include the following:

  • Neural tube defect: The neural tube becomes the baby's brain and spine. If the tube doesn't close completely before birth, it can result in spina bifida or anencephaly.
  • Abdominal wall defects: The muscle and skin that cover the wall of the abdomen do not form as they should, allowing the bowel to be exposed through the abdominal wall. This is called gastroschisis. Another type of abdominal defect, omphalocele, occurs when the tissue around the umbilical cord is so weak that organs can protrude through it.
  • Heart defect: The chambers of the heart are not properly formed.
  • Down syndrome: The baby inherits an extra chromosome 21 that results in severe learning disabilities and retardation, abnormal facial features, and various medical conditions.
  • Trisomy 18: The baby inherits an extra chromosome 18, which causes extreme retardation, birth defects, and may even cause death.

What are genes and chromosomes?

Genes and chromosomes are the way a baby inherits traits. All of the cells in the body contain genes and chromosomes. Each chromosome has many genes. The fetus gets half of its genes from its mother and the other half from its father. Some traits are determined by only one gene, while others are caused by  a combination of genes. 

A gene or a genetic disorder can be either dominant or recessive. If one gene in the pair the baby receives is dominant, the trait it carries overrides the trait carried by the recessive gene. Traits carried by recessive genes can only be inherited if both parents contribute that recessive gene to the baby.

Some ethnic groups have a tendency to pass on certain genetic disorders. A screening test called carrier testing can be performed before, after, or during pregnancy to determine the risk of some, but not all, of these disorders. 

When should testing for birth defects be performed?

All pregnant women have the option to undergo screening tests to determine their risk of having a baby with a birth defect or genetic disorder. If the results of the screening test indicate there is a high risk of having a baby with these types of defects, the obstetrician will order additional tests to diagnose the problem. An abnormal screening test result is only an indication that there may be a problem. In most cases, the baby doesn't have a medical issue, even if there is an abnormal screening test result. Similarly, a baby can be born with a birth defect even if the test result indicates there is no birth defect present. 

Women with high-risk pregnancies for birth defects may undergo a diagnostic test first rather than having a screening test. These pregnancy risk factors may include the following:

  • A family history of birth defects
  • A previous pregnancy resulting in the birth of a child with a birth defect or genetic condition
  • Taking particular medicines around the time of conception
  • Having diabetes before becoming pregnant

What is carrier testing?

A carrier is someone who doesn't appear to have a particular medical condition but could pass the gene for this condition on to his or her children. Carrier testing is performed to determine if a couple carries genes for certain inherited disorders. Some of these genetic conditions include cystic fibrosis, sickle cell anemia, Tay–Sachs disease, thalassemia, familial dysautonomia, and Canavan disease.

A blood or saliva sample is studied in a lab. If the test result shows the mother is a carrier, the father is then tested. If the test result shows that both parents are carriers, a genetic counselor will discuss the risk of having a baby with the medical condition.

What are the types of screening tests?

Screening tests do not harm the fetus. Many tests are available, and they are performed based on the trimester of pregnancy. Women can choose to have a single test or a combination. Some of these tests may not be available everywhere.

Your obstetrician will discuss the risks and benefits of the screening tests to help you decide. If you have a family history of birth defects, your doctor may suggest visiting a genetic counselor for more detailed information about your risks. 

What screening tests are performed during the first trimester?

During the first trimester, pregnant women undergo blood tests and an ultrasound exam. They can be performed individually or as a combination test. First trimester screening is performed between week 11 and week 14 of pregnancy to determine the risk of Down syndrome and trisomy 18. The blood tests measures:

  1. Pregnancy-associated plasma protein-A (PAPP-A)
  2. Human chorionic gonadotropin (hCG)

An ultrasound exam, also known as nuchal translucency screening, measures the thickness at the back of the neck of the fetus. A larger-than-normal thickness may be a sign of Down syndrome, trisomy 18, or other chromosomal problems.

The results of the nuchal translucency screening are then assessed in combination with the blood tests, and the mother's age to determine the risk for birth defects. In the first trimester, this combined evaluation detects Down syndrome in 82 to 87 percent of cases. Increased thickness in the nuchal translucency may also be a sign that the fetus may have a heart defect. In this case, your obstetrician may recommend a more detailed ultrasound exam around 20 weeks of pregnancy.

What is a detailed ultrasound exam?

This type of exam shows a more complex view of the baby's organs and features. A detailed ultrasound exam can be performed after 18 weeks of pregnancy. A detailed ultrasound cannot detect all birth defects.

What screening tests are performed during the second trimester?

A "multiple marker screening" is conducted to screen for Down syndrome, trisomy 18, and neural tube defects. This test measures the following substances in your blood:

  • Alpha-fetoprotein (AFP)— Found in amniotic fluid, fetal blood, and, to some degree, in the mother's blood
  • Estriol—A hormone created by the placenta and the fetus' liver
  • Human chorionic gonadotropin—A hormone created by the placenta
  • Inhibin-A—A hormone created by the placenta

The triple screen, which measures levels of the first three substances, detects Down syndrome in 69 percent of cases. The quad screen, which tests the levels of all four substances, detects Down syndrome in 81 percent of cases. Testing AFP detects neural tube defects in 80 percent of cases. These tests are typically performed between week 15 and week 20 of pregnancy. The timing of the tests is important because levels of these substances fluctuate during pregnancy.

The results from both first and second trimester tests are combined to increase their effectiveness in detecting Down syndrome. When both the first and second trimester tests are assessed in combination, about 90 to 95 percent of Down syndrome cases can be detected.

What happens if the test results indicate problems?

If the screening test raises concerns about your pregnancy, additional diagnostic tests can be performed to provide more information:

  • Detailed ultrasound exam—A type of ultrasound that provides more extensive information about the development of the fetus
  • Amniocentesis—A small amount of amniotic fluid and cells are removed from the sac surrounding the fetus and tested
  • Chorionic villus sampling (CVS)—A small sample of cells from the placenta is tested 

Screening tests aid in the determination of risk for a birth defect. Talk to your doctor about which screening tests you should undergo. 

Glossary 

Anencephaly: A type of neural tube defect in which the baby is born with parts of the brain and skull missing.

Canavan Disease: A rare disorder in which the brain breaks down. These children usually die before the age of four, although some children may survive into their teens and twenties.

Cystic Fibrosis: An incurable condition that causes problems with digestion and breathing.

Diabetes: A condition in which the body is unable to metabolize sugar properly.

Human Chorionic Gonadotropin (hCG): A hormone produced during pregnancy. Testing urine for hCG determines if someone is pregnant.

Nuchal Translucency Screening: An advanced ultrasound test to screen for the risk of Down syndrome and other birth defects.

Placenta: The mechanism through which the fetus receives nourishment and its wastes are removed.

Sickle Cell Anemia: A blood disorder in which the red blood cells are improperly formed and have a crescent, or “sickle,” shape. These cells do not allow proper flow of oxygen to organs and tissues, which causes pain.

Tay–Sachs Disease: Toxic amounts of a fatty substance called ganglioside GM2 gather in the nerve cells in the brain, causing severe mental retardation, blindness, and seizures.

Trimester: One of the three stages into which pregnancy is divided. A trimester lasts for three months.

Ultrasound Exam: A test in which sound waves are sent into the body and the echoes the waves make are used to create a “picture” of internal structures. During pregnancy, it can be used to examine the fetus.

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