Diagnosing Birth Defects

Prenatal tests can be done to detect certain birth defects. Screening tests can help detect the risk of a possible problem. Based on the results, some pregnant women may be offered a diagnostic test.

Birth Defects
Almost every mother-to-be worries about her baby having a problem. Most of the time, this worry is needless. Almost all children in the United States are born healthy. Out of 100 newborns, only two or three have major birth defects. A birth defect is a mental or physical problem that is present at birth. It also is called a congenital disorder. Birth defects can be genetic, occurring in genes or chromosomes, or caused by exposure to harmful agents.

Risk Factors
Many babies with birth defects are born to couples with no risk factors. However, the risk of birth defects is increased when certain factors are present:

  1. Family or personal history of birth defects
  2. Previous child with a birth defect
  3. Certain medicines used around the time of conception
  4. Diabetes before pregnancy
  5. Woman aged 35 years or older when the baby is due

Screening Tests
Screening tests are done during pregnancy to assess the risk of certain birth defects. These tests do not diagnose birth defects. There are several types of screening tests:

  1. First trimester screening. This screening test combines the results of a special ultrasound tests and blood tests to detect Down syndrome and trisomy 18.
  2. Maternal serum screening. A group of blood (serum) tests that check for abnormal levels of substances linked with certain birth defects, such as Down syndrome and neural tube defects.
  3. Detailed ultrasound exam. This type of ultrasound exam is usually done after 18 weeks of pregnancy and allows a more extensive view of the baby’s organs and features.

Diagnostic Tests
Diagnostic tests are done to detect a genetic disorder or birth defect. They are offered to women who are at risk of a genetic disorder based on their family histories or the results of screening tests.

Detailed Ultrasound Exam
During pregnancy, many women have a basic ultrasound exam. A detailed ultrasound exam allows a more extensive view of the baby’s organs and features. It can help explain abnormal screening test results and provide more information.

Amniocentesis usually is done at 15-20 weeks of pregnancy. To perform the procedure, a doctor guides a thin needle through your abdomen and uterus. A small sample of amniotic fluid is withdrawn and sent to a lab.

In the lab, cells from the baby that were taken from the amniotic fluid are grown in a special culture. This usually takes about 10-12 days. Next, the chromosomes in these cells are studied under a microscope. This shows if there is an extra chromosome (as in Down syndrome) or other chromosomal defects.

Complications from amniocentesis are uncommon. Side effects may include cramping, vaginal bleeding, infection and leaking amniotic fluid.

Chorionic Villus Sampling
Chorionic villus sampling (CVS) detects some of the same chromosomal problems as amniocentesis does. It is performed earlier than amniocentesis at 10-12 weeks of pregnancy.

Fetal Blood Sampling
Also known as cordocentesis, fetal blood sampling is used to test for chromosomal defects and other problems.

The Next Steps
Most of the time, the results of a diagnostic test are normal. If diagnostic tests show that there is a problem, a counselor can help explain how the baby will be affected.

Finally . . .
If you receive abnormal results from a screening test or are at increased risk for having a baby with a birth defect, you may be offered diagnostic testing during pregnancy. Most of the time, the results of a test will confirm that the baby is healthy. If the results are abnormal, knowing about problems before the baby is born may help a woman make decisions about health care for her baby.

This excerpt from ACOG’s Patient Education Pamphlet is provided for your information. It is not medical advice and should not be relied upon as a substitute for visiting your doctor. If you need medical care, have any questions, or wish to receive the full text of this Patient Education Pamphlet, please contact your obstetrician-gynecologist.

To ensure the information is current and accurate, ACOG titles are reviewed every 18 months.

Copyright © April 2005 The American College of Obstetricians and Gynecologists