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. For about 70% of babies born with birth defects, the cause is not known.

Birth defects are caused by an error in the way the heart, kidney, bone, muscles, brain, skin, or other organs developed. A birth defect may affect how the body looks, works, or both. Many birth defects are mild, but some can be severe. Although it is rare, some birth defects can cause the baby to die. Babies with birth defects may need surgery or other medical treatments.

Most birth defects occur during the first 3 months of pregnancy. Some birth defects may be found before birth with special tests. Others appear at birth or later in a person’s life. Some of the most common birth defects found through prenatal testing include:

• Neural tube defect: Incomplete closure of the fetal spine that can result in spina bifida or anencephaly.
• Abdominal wall defects: One type of defect occurs when the muscle and skin that cover the wall of the abdomen are missing and the bowel sticks out through a hole in the abdominal wall (gastroschisis). Another type is when the tissue around the umbilical cord is weak and allows organs to protrude into this area (omphalocele).
• Heart defect: The chambers or pathways through the heart are not properly developed.
• Down syndrome: Mental retardation, abnormal features of the face, and medical problems such as heart defects occur because of an extra chromosome.
• Trisomy 18: There is an extra chromosome 18, which causes severe problems with growth and development.

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:

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

Carrier testing also can be done before, during, and after pregnancy to see if a woman is at risk for certain genetic disorders, including cystic fibrosis and sickle cell anemia. If the test results show that both parents are carriers, a genetic counselor can provide more information about the risk of having a baby with the disorder. Diagnostic testing can show if the defect is present.

If a woman is at risk for a specific disorder—such as cystic fibrosis—the cells of the fetus can be tested by a procedure, such as amniocentesis, to see if the baby will have this condition. However, such testing is done only if a woman’s history or previous test results suggest she is at increased risk.

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:

• First trimester screening. This screening test combines the results of a special ultrasound tests and blood tests to detect Down syndrome and trisomy 18.
• 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.
• 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. Diagnostic tests include a detailed ultrasound exam, amniocentesis, and chorionic villus sampling. These exams may need to be done in a special center equipped to perform them.

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.

A detailed ultrasound exam can show many birth defects. If heart problems are suspected in the fetus, a fetal echocardiogram (another type of ultrasound exam) may be done to examine the baby’s heart in more detail. Many heart defects may be detected by ultrasound, but some may not. In some cases, other diagnostic tests are needed to provide more information.

Amniocentesis

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. The alpha-fetoprotein (AFP) level in the amniotic fluid also can be tested to help determine if the fetus has a neural tube defect.

Complications from amniocentesis are uncommon. Side effects may include cramping, vaginal bleeding, infection, and leaking amniotic fluid. There is a slight chance of miscarriage (less than 1%) as a result of amniocentesis.

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.

To perform CVS, a doctor guides either a small tube through your vagina and cervix (transcervical) or a thin needle through your abdomen and uterine wall (transabdominal). The doctor then takes a small sample of tissue from the placenta. The chorionic villi contain cells with the same genetic makeup as the fetus.

The sample is sent to a lab, where it is grown in a culture. This usually takes about 7 days. The sample is then studied under a microscope to check for chromosomal or other certain defects.

Complications from CVS are uncommon. Chorionic villus sampling carries a slightly increased risk of miscarriage than amniocentesis (about 1%).

Fetal Blood Sampling

Also known as cordocentesis, fetal blood sampling is used to test for chromosomal defects and other problems. For this test, a needle is placed through the abdomen and uterine wall and blood is taken from a vein in the umbilical cord. Fetal blood sampling usually is used when amniocentesis or chorionic villus sampling is not possible because results are needed more quickly to clarify the results of previous tests. This test may be done at 18 weeks of pregnancy or later.

As with amniocentesis, possible side effects from this procedure are infection, cramping, and bleeding. There is a small (1–2%) chance of fetal loss after fetal blood sampling.

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. In some cases, it is not known how severe a birth defect will be. Sometimes surgery or treatment is possible. In other cases, there is no treatment for a defect. The counselor and doctor can explain the results and provide guidance in making choices and considering options. Testing can help prepare you for the future so you can plan for any special care that may be needed.

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. Talk to your doctor about the risks and benefits of diagnostic testing.

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.

Amniotic Fluid: Water in the sac surrounding the fetus in the mother’s uterus.

Anencephaly: A type of neural tube defect that occurs when the fetus’s head and brain do not develop normally.

Chromosome: A structure that is located inside each cell in the body and contains the genes that determine a person’s physical makeup.

Genes: DNA “blueprints” that code for specific traits, such as hair and eye color.

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

Spina Bifida: A neural tube defect that results from incomplete closure of the fetal spine.

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

This Patient Education Pamphlet was developed under the direction of the Committee on Patient Education of the American College of Obstetricians and Gynecologists. Designed as an aid to patients, it sets forth current information and opinions on subjects related to women’s health. The average readability level of the series, based on the Fry formula, is grade 6–8. The Suitability Assessment of Materials (SAM) instrument rates the pamphlets as “superior.” To ensure the information is current and accurate, the pamphlets are reviewed every 18 months. The information in this pamphlet 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 2005 by the American College of Obstetricians and Gynecologists. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher.

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