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Amniocentesis is the most common prenatal test used to diagnose chromosomal and genetic birth defects. Another prenatal test, called chorionic villus sampling (CVS), can diagnose most of the same birth defects as amniocentesis. CVS is done earlier in pregnancy (usually between 10 and 12 weeks) than amniocentesis (usually 15 to 20 weeks), but appears to pose a slightly higher risk of miscarriage (2, 3).
Who is offered amniocentesis?
Health care providers do not routinely offer amniocentesis to all pregnant women because it carries a small risk of miscarriage. They offer amniocentesis when there is an increased risk of chromosomal or genetic birth defects or certain malformations.
Amniocentesis may be offered because of:
- Maternal age: The risk of bearing children with certain chromosomal birth defects increases as a woman ages. If a woman will be 35 or older at the time of delivery, health care providers offer the option of prenatal diagnostic testing for chromosomal disorders. The most common of these disorders is Down syndrome, a combination of mental and physical abnormalities caused by an extra chromosome. Down syndrome occurs in about 1 in 1,250 children born to women in their 20s. The chances increase to about 1 in 400 by age 35, 1 in 100 at age 40, and 1 in 30 at age 45 (2).
- A previous child or pregnancy with a birth defect: If a couple already has had a child (or pregnancy) diagnosed with a chromosomal abnormality, a genetic birth defect, or certain birth defects involving the brain or spinal cord (neural tube defects or NTDs), the couple may be offered prenatal testing during subsequent pregnancies.
- Abnormal screening test results: Screening tests for birth defects can be done in the first or second trimester. The first-trimester screening test combines a blood test with an ultrasound exam to measure the skin fold at the back of the fetal neck. The second trimester screening test also can suggest an increased risk for neural tube defects. Abnormal results on either the first- or second-trimester screening test can suggest an increased risk of Down syndrome and certain other chromosomal abnormalities. Amniocentesis can diagnose or rule out chromosomal abnormalities and can help diagnose NTDs.
- Family history: Couples with a family history of certain birth defects, such as cystic fibrosis (lung disease) or fragile X syndrome (mental retardation), may be offered amniocentesis.
When is amniocentesis done?
Amniocentesis usually is done in the second trimester between 15 and 20 weeks of pregnancy. Some medical centers offer early amniocentesis, done in the first trimester, between 11 and 14 weeks after the last menstrual period. However, recent studies suggest that early amniocentesis is riskier than second-trimester amniocentesis (4).
Amniocentesis also can be used in the third trimester to diagnose uterine infections and to determine whether the fetus's lungs are mature enough for delivery (in cases where early delivery may be necessary). The test also helps evaluate the severity of anemia in fetuses with Rh disease (an incompatibility between the blood types of the mother and baby). This information helps the health care provider determine whether a fetus with Rh disease requires lifesaving blood transfusions.
How is amniocentesis performed?
During the procedure, the pregnant woman lies flat on her back on a table. The doctor cleanses her belly with an antibacterial solution. Then, using ultrasound (a test that uses sound waves to take a picture of the fetus) to guide her, the doctor inserts a thin needle through the abdomen and uterus into the amniotic sac. She then withdraws about 1 to 2 tablespoons of fluid and removes the needle. After the sample is taken, the doctor uses ultrasound to check that the fetal heartbeat is normal. The entire procedure takes just a few minutes.
Some women say that amniocentesis doesn't hurt at all. Others feel cramping when the needle enters the uterus or pressure when the fluid is being withdrawn. One to 2 percent of women experience cramping, spotting or leakage of amniotic fluid after the procedure (4). After the test, most health care providers recommend that a woman avoid strenuous activity and take it easy for the rest of the day.
What happens after the fluid is removed?
Skin cells from the fetus float in the amniotic fluid. After a sample of amniotic fluid is removed, these cells are grown in a laboratory for 10 to 12 days, then tested for chromosomal abnormalities or genetic birth defects (5). Test results usually are available within three weeks (2).
The levels of a protein called alpha-fetoprotein (AFP) in the amniotic fluid also can be measured. This protein is made by the fetus and is often elevated in the case of an NTD. Results from this test, which help diagnose NTDs, may take only a few days.
Is amniocentesis safe?
Serious complications from second-trimester amniocentesis are uncommon. However, the procedure does pose a small risk of miscarriage. According to the Centers for Disease Control and Prevention (CDC), between one in 400 and one in 200 women have a miscarriage after amniocentesis (3). Other complications, including injury to the fetus and infection, are rare (2, 3).
Studies suggest that the risk of miscarriage after first-trimester amniocentesis may be 3 times higher than the risk after second-trimester amniocentesis (6). Studies also suggest that early amniocentesis may increase the risk of a foot deformity called clubfoot (6, 7). Studies suggest that, if first-trimester prenatal testing is necessary, chorionic villus sampling appears safer than early amniocentesis (7).
The risk of pregnancy loss following amniocentesis is lower when the physician performing the procedure is highly experienced. Experienced doctors often practice at major medical centers. Health care providers and genetic counselors usually can provide pregnant women with referrals to experienced physicians.
Do normal amniocentesis results mean a baby will be born healthy?
Most women who have amniocentesis receive reassuring news that their fetus does not have the disorders for which it was tested. However, no prenatal test can guarantee the birth of a healthy baby. About 3 out of every 100 babies have a birth defect (8).
Can health care providers treat the birth defects diagnosed by amniocentesis?
Health care providers can treat some birth defects diagnosed before birth using medications, dietary treatments or surgery. For example, doctors have diagnosed and treated biotin deficiency and methylmalonic acidemia (life-threatening inherited disorders of body chemistry) before birth, resulting in the birth of healthy babies (9). In most cases, however, birth defects diagnosed by amniocentesis cannot be treated before birth.
If a fetus has a birth defect that cannot be treated before birth, prenatal diagnosis may help parents prepare emotionally for the birth. The parents also can plan the delivery with their health care provider, so the baby can have any necessary special care right after birth. Parents can discuss their options with genetic counselors as well as with their health care providers.
How can a woman decide if amniocentesis is right for her?
Whether or not to have amniocentesis is a matter for discussion between parents and health professionals. Genetic counselors, physicians, and religious and ethical counselors can be valuable in helping parents make decisions about prenatal diagnosis and other reproductive issues.
Couples deciding between CVS and amniocentesis need to consider many factors, including the technical expertise available, a woman's medical history and preferences, and the condition diagnosed. With one main exception (CVS cannot diagnose NTDs), CVS and amniocentesis test for the same birth defects. The procedures differ slightly, however, in risk, timing and how soon results are available.
Some women over age 35 may choose to have a first- or second-trimester screening test to get more information on their risk of having a baby with Down syndrome and other chromosomal birth defects before deciding whether or not to go ahead with amniocentesis. If the screening test shows a woman is at low risk of Down syndrome, she may choose not to have amniocentesis. However, the screening tests cannot definitely diagnose or rule out Down syndrome and other chromosomal birth defects as amniocentesis or CVS can.
The decision to have amniocentesis requires careful planning and thoughtful discussion. A woman should discuss this issue with her health care provider during an early prenatal visit or even a pre-pregnancy visit.
Are there ways to reduce the risk of birth defects?
All women can do some basic things to improve their chances of having a healthy pregnancy and a healthy baby:
- Have a pre-pregnancy checkup with a health care provider.
- Get early and regular prenatal care.
- Take a multivitamin containing 400 micrograms of folic acid daily starting before pregnancy and in early pregnancy to help prevent neural tube defects.
- Eat a variety of nutritious foods, including foods containing folic acid, like fortified breakfast cereals, leafy green vegetables, dried beans, legumes, oranges and orange juice.
- Don't eat undercooked meat or change a cat's litter box. Both are possible sources of toxoplasmosis, an infection that can cause birth defects.
- Don't eat shark, swordfish, king mackerel or tilefish. These fish can have high amounts of mercury. It's all right for a pregnant woman to eat a limited amount of fish that have small amounts of mercury, including shrimp, salmon, pollock, catfish and canned light tuna. But she should not eat more than 6 ounces of albacore (white) tuna per week.Women also should check local advisories about the safety of fish caught in local waters.
- Begin pregnancy at a healthy weight (not too heavy or too thin).
- Don't smoke and avoid secondhand smoke.
- Don't drink alcohol.
- Don't use any drug, even over-the-counter medications or herbal preparations, unless recommended by a health care provider who knows you are pregnant.
In addition, the March of Dimes recommends that all women be tested for immunity to rubella (German measles) and chickenpox before becoming pregnant and consider being vaccinated if they are not immune. After being vaccinated, a woman should wait one month before becoming pregnant.
References
1. National Institutes of Health. Antenatal Diagnosis: Report of a Consensus Development Conference. NIH Publication No. 79-1973, April 1979.
2. American College of Obstetricians and Gynecologists (ACOG). Your Pregnancy and Birth, 4th Edition. ACOG, Washington, DC, 2005.
3. Centers for Disease Control and Prevention (CDC). Chorionic Villus Sampling and Amniocentesis: Recommendations for Prenatal Counseling. Morbidity and Mortality Weekly Report, volume 44, number RR-9, July 21, 1995.
4. American College of Obstetricians and Gynecologists (ACOG). Prenatal Diagnosis of Fetal Chromosomal Abnormalities. ACOG Practice Bulletin, number 27, May 2001.
5. American College of Obstetricians and Gynecologists (ACOG). Diagnosing Birth Defects. ACOG Educational Pamphlet, number AP164, April 2005.
6. Canadian Early and Mid-Trimester Amniocentesis Trial (CEMAT) Group. Randomized Trial to Assess Safety and Fetal Outcome of Early and Midtrimester Amniocentesis. The Lancet, volume 351, January 24, 1998, pages 242-247.
7. Philip. J., et al. Late First-Trimester Invasive Prenatal Diagnosis: Results of an International Randomized Trial. Obstetrics and Gynecology, volume 103, number 6, June 2004, pages 1164-1173.
8. Centers for Disease Control and Prevention (CDC). Birth Defects: Frequently Asked Questions. October 29, 2004.
9. Harrison, M.R., et al. Management of the Fetus with a Correctable Congenital Defect. Journal of the American Medical Association, 1981, volume 246, number 7, pages 774-777.
August 2005 (R 2-07, 6-07)
