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Amniocentesis is a common prenatal test in which a small sample of the amniotic fluid surrounding the fetus is removed and examined. This test is used to diagnose or, far more likely, rule out certain birth defects and genetic disorders.
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 CVS may pose a slightly higher risk of miscarriage than amniocentesis (though recent studies suggest that risk may be similar when the procedures are done by experienced doctors) (1).
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.
Providers may offer amniocentesis because of:
- Maternal age: The risk of bearing children with certain chromosomal birth defects increases as a woman ages. Traditionally, providers have offered women who will be 35 or older at the time of delivery 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 (1).
Today women over age 35 generally choose to have a first- or second-trimester screening test to get more information on their risk before deciding whether or not to have amniocentesis. The American College of Obstetricians and Gynecologists (ACOG) recommends that first- and second-trimester screening as well as prenatal testing be available to women of all ages (1).
- A previous child or pregnancy with a birth defect: If a woman 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), she 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 to measure the skin fold (called the nuchal fold) at the back of the fetal neck. An ultrasound uses sound waves to take a picture of the fetus.
- First-trimester screening helps determine a woman’s risk for having a baby with Down syndrome or trisomy 18 (a severe chromosomal abnormality).
- The second-trimester screening test is a blood test that helps determine the risk of the same chromosomal birth defects, as well as for NTDs.
If a screening test suggests an increased risk of birth defects, providers often recommend amniocentesis to diagnose or rule out chromosomal abnormalities and to help diagnose NTDs. Most babies with abnormal screening test results are healthy.
- Family history: Providers may offer amniocentesis to couples with a family history of certain birth defects and genetic disorders, such as cystic fibrosis or fragile X syndrome.
When is amniocentesis done?
Amniocentesis usually is done in the second trimester between 15 and 20 weeks of pregnancy. Early amniocentesis (done between 11 and 14 weeks after the last menstrual period) is no longer recommended because it poses a higher risk of miscarriage and other complications than second-trimester amniocentesis (1).
Amniocentesis also can be used in the third trimester to diagnose uterine infections and to determine whether the fetus’ lungs are mature enough for delivery (in cases where early delivery is being considered). 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?
First, the doctor does an ultrasound to locate the fetus, the placenta and pockets of amniotic fluid from which to take a sample. Then the doctor cleans the woman’s belly with an antibacterial solution. During the procedure, the pregnant woman lies flat on her back on a table.
Using ultrasound to guide him, the doctor inserts a thin needle through the abdomen and uterus into the amniotic sac. He 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 (1). 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 to the fluid that is removed?
Both the fluid and skin cells from the fetus that float in the amniotic fluid can be analyzed. Cells are separated from the amniotic fluid and grown in a laboratory for 10 to 12 days. Then the cells are tested for chromosomal abnormalities or genetic birth defects. Test results usually are available within 2 weeks (1).
The levels of a protein called alpha-fetoprotein (AFP) in the amniotic fluid can be measured to help diagnose NTDs. AFP levels are often increased when the baby has an NTD. Results from this test are usually available in 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 ACOG, fewer than 1 in 500 to 1 in 300 women have a miscarriage after amniocentesis (1). One recent study suggests that the risk of miscarriage may be as low as 1 in 1,600 (2). Other complications, such as uterine infections, are rare, occurring in fewer than 1 in 1,000 cases (1).
Amniocentesis in the first trimester is not recommended (1). Studies suggest that the risk of miscarriage after first-trimester amniocentesis may be 3 times higher than the risk after second-trimester amniocentesis (1, 3). Studies also suggest that early amniocentesis may increase the risk of a foot deformity called clubfoot (1, 3). When first-trimester prenatal testing is necessary, CVS is a safer option.
The risk of pregnancy loss following amniocentesis may be lower when the doctor performing the procedure is highly experienced (1). Experienced doctors often practice at major medical centers. Health care providers and genetic counselors usually can provide pregnant women with referrals to experienced doctors.
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.
Can health care providers treat the birth defects diagnosed by amniocentesis?
Health care providers can treat a small number of 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 (4). 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 care providers. Genetic counselors, doctors, 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 timing, how soon results are available and, possibly, in risk.
If a screening test shows a low risk of Down syndrome, some women may choose not to go ahead with amniocentesis. However, in making this decision, it’s important to keep in mind that 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 preconception 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 preconception 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 NTDs.
- Eat healthy foods, including foods containing folic acid and folate (the form of folic acid that occurs naturally in foods), like fortified breakfast cereals, enriched grain products, beans, leafy green vegetables 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.
- Avoid contact with all rodents, including pet hamsters, mice and guinea pigs. These animals can carry a virus that can harm a baby.
- Avoid mercury in fish. 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 up to 12 ounces a week of fish that have small amounts of mercury, such as 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 the woman is 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 1 month before becoming pregnant.
References
- American College of Obstetricians and Gynecologists (ACOG). Invasive Prenatal Testing for Aneuploidy. ACOG Practice Bulletin, number 88, December 2007.
- Eddleman, K.A., et al. Pregnancy Loss Rates After Midtrimester Amniocentesis. Obstetrics and Gynecology, volume 108, number 5, November 2006, pages 1067-1072.
- 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.
- Natarajan, G. The Fetus as a Patient: Prenatal Diagnosis and Fetal Therapy. Emedicine, updated 8/22/06.
August 2008
