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| Genetics->Testing: Amniocentesis carries a small risk of miscarriage |
Amniocentesis is a common prenatal test in which a small sample of the amniotic fluid surrounding the fetus is removed and examined.
It was first used in 1882 to remove excess amniotic fluid, and has long been performed in late pregnancy to assess anemia in babies with Rh disease and to find out if the fetal lungs are mature enough for the baby to be delivered.
Today, amniocentesis often is used in the second trimester of pregnancy (usually 15 to 18 weeks after a woman’s last menstrual period) to diagnose or, far more likely, rule out certain birth defects.
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, but not all, of the same birth defects as amniocentesis.
CVS is done earlier in pregnancy (usually between 10 and 12 weeks) than amniocentesis, but appears to pose a slightly higher risk of miscarriage and other complications. Some studies also suggest that CVS may pose an extremely small risk of birth defects involving the fingers and toes.
Amniocentesis isn't a routine test
Amniocentesis is not routinely offered to all pregnant women because it carries a small risk of miscarriage. Amniocentesis is offered 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, most physicians offer the option of prenatal testing for chromosomal disorders.
The most common of these disorders is Down syndrome, a combination of mental and physical abnormalities caused by the presence of an extra chromosome. Down syndrome occurs in approximately one in 1,250 children born to women in their 20s.
The chances increase to one in 400 by age 35, and one in 100 at age 40.
A previous child or pregnancy with a birth defect. If a couple already has had a child (or pregnancy) diagnosed with a chromosomal abnormality, any of a wide range of genetic birth defects, or a neural tube defect (see below), the couple may be offered prenatal testing during subsequent pregnancies.
Suggestive screening test results. It is increasingly common to screen pregnant women by testing their blood for alpha fetoprotein (AFP) and certain other substances.
A high level of AFP suggests a fetus with a neural tube defect (malformation of the spinal cord or brain, such as spina bifida or anencephaly).
A low level of AFP and variations in the other substances suggest a chromosome abnormality. Amniocentesis can help detect neural tube defects by measuring AFP in amniotic fluid, and can diagnose most chromosomal abnormalities.
Other family history. Couples without a previously affected child also may be offered prenatal testing if their family medical histories indicate their children may be at increased risk of inheriting a genetic disorder. Prenatal diagnosis is possible for virtually all chromosomal disorders, but not all genetic ones.
When is amniocentesis done?
Amniocentesis usually is done in the second trimester. Some medical centers offer early amniocentesis, done between 11 and 14 weeks after the last menstrual period.
However, early amniocentesis is considered experimental and recent studies suggest that it is riskier than second-trimester amniocentesis (see below).
There also are several uses for amniocentesis in the third trimester. Beside determining whether the fetus’s lungs are mature enough for delivery in cases where early delivery may be necessary, amniocentesis can diagnose uterine infections and may be recommended if a pregnant woman’s membranes have ruptured prematurely.
The test also can determine the severity of fetal anemia in babies with Rh disease and help the doctor determine whether the fetus requires lifesaving blood transfusions.
Amniocentesis is performed with a hollow needle
Amniocentesis is performed by inserting a thin, hollow needle into the uterus and removing some of the amniotic fluid that surrounds the baby. During the procedure, the pregnant woman lies flat on her back on a table. H
er belly is cleansed with an iodine solution and the physician, using ultrasound to guide her, inserts a thin needle through the abdomen and uterus into the amniotic sac.
She then withdraws about one to two tablespoons of fluid and removes the needle. After the sample is taken, the physician 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 during the short time the fluid is being withdrawn.
One to two percent of women experience cramping, spotting, or leakage of amniotic fluid after the procedure. Most physicians recommend that a woman take it easy for several hours after amniocentesis, avoiding physical stresses such as lifting and prolonged standing.
What happens after the fluid is removed?
Living 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 one to two weeks, then tested for chromosomal abnormalities or various genetic birth defects.
Test results usually are available within 3 weeks. Because AFP can be measured directly, without waiting for cells to grow, results of this test may take only a few days.
Is amniocentesis safe?
Millions of women have had prenatal diagnosis by amniocentesis. In 1976, after careful study, the National Institutes of Health reported that it found midtrimester amniocentesis for prenatal diagnosis to be safe. However, amniocentesis does pose a slight risk of miscarriage.
According to the Centers for Disease Control and Prevention (CDC), the rate of miscarriage is between one in 400 and one in 200 procedures. The procedure also carries an extremely low risk of uterine infection (less than one in 1,000), which can cause miscarriage.
Studies suggest that the risk of miscarriage following first-trimester amniocentesis may be three times higher than the risk after second-trimester amniocentesis.
A 1998 Canadian study found the risk of miscarriage was 2.6 percent after early amniocentesis, compared to 0.8 percent after second-trimester amniocentesis. The study also found a striking increase in the risk of a foot deformity called clubfoot after early amniocentesis.
The risk of clubfoot was increased ten-fold after early amniocentesis (1.3 percent vs. 0.1 percent or 1 in 1,000 following second-trimester amniocentesis). The incidence of clubfoot following second-trimester amniocentesis does not differ from that seen in all U.S. babies.
Based upon this and other studies, doctors are rethinking the role of early amniocentesis, and many believe that if first-trimester prenatal testing is necessary, chorionic villus sampling appears safer than early amniocentesis.
The risk of pregnancy loss following amniocentesis is lower when the physician performing the procedure is highly experienced. Experienced doctors often are located 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?
More than 95 percent of the high-risk women who have prenatal diagnosis receive reassuring news that their unborn babies do not have the disorders for which they are tested.
However, no one prenatal test can guarantee the birth of a healthy baby, since only some birth defects can be ruled out before birth.
Three to four out of every 100 babies have a birth defect. Amniocentesis has an accuracy rate of between 99.4 and 100 percent in diagnosing chromosomal abnormalities.
Can doctors treat the birth defects diagnosed by amniocentesis?
Currently, physicians are able to diagnose many more birth defects than they are able to treat prenatally. However, advances in prenatal therapy now make it possible to treat some birth defects before birth.
For example, biotin dependence and MMA (methylmalonic acidemia), two life-threatening inherited disorders of body chemistry, have been detected by amniocentesis and treated in the womb, resulting in the birth of healthy babies.
If a fetus has a condition for which prenatal treatment is not yet possible, prenatal diagnosis may help parents to prepare emotionally for the birth and to plan the delivery with their health care provider.
Parents can discuss their options with genetic counselors as well as with their health care providers.
Who should have amniocentesis?
Whether or not to have prenatal diagnosis is a matter for individual 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 decisions.
Couples deciding between CVS and amniocentesis need to consider many factors including the technical expertise available, a woman’s medical history and preferences, and what condition is being diagnosed.
With one main exception (CVS cannot diagnose neural tube defects), CVS and amniocentesis test for the same birth defects. The procedures differ slightly, however, in risk, timing, reliability and how soon results are available.
Some diagnostic tests such as CVS are performed early in pregnancy, and the decision to go ahead with the tests or not requires careful planning and thoughtful discussion.
This is just one more reason why it is important that all women get early prenatal care, and why visiting a health care provider before conception is strongly recommended.
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