Amniocentesis and CVS

Definition

Amniocentesis (amnio) and chorionic villus sampling (CVS) are prenatal tests that can diagnose - or, far more often - rule out, certain birth defects.

The two procedures test for the same birth defects, except neural tube defects such as spina bifida, which only CVS detects.

Amniocentesis

Amniocentesis is a simple medical procedure used to obtain small samples of the amniotic fluid surrounding the fetus. It was developed in the 1800s, and has long been used late in pregnancy to assess fetal anemia in Rh disease and to find out if the fetal lungs are mature enough for delivery. Today, amniocentesis is often used in the second trimester of pregnancy to diagnose certain birth defects, or in most cases their absence.

Amniocentesis is not routinely offered during pregnancy because the test itself carries a small risk of infection or miscarriage. Amniocentesis is offered as an optional test when there is an increased risk of chromosomal or genetic birth defects, or certain malformations.

Amniocentesis may be recommended because of:

Age. The risk of bearing children with 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. Among the most common of these disorders is Down syndrome, a combination of mental and physical abnormalities caused by the presence of an extra chromosome.

A previous child or pregnancy with a birth defect. If a couple already has a child (or pregnancy) diagnosed with a chromosomal abnormality, a biochemical birth defect, or a neural tube defect, the couple may be offered prenatal testing during subsequent pregnancies.

Other family history. Couples without a previously affected child may also be offered prenatal testing if their family medical histories indicate their children may be at increased risk of inheriting a genetic disorder. One or both prospective parents may be "carriers" of a disorder, or a disorder may "run in the family." Prenatal testing would be done only if the suspected condition can be diagnosed prenatally.

Suspected neural tube defects. These defects of the spine and brain, including spina bifida and anencephaly, can be diagnosed by measuring the level of alphafetoprotein (AFP) in the amniotic fluid. Amniocentesis to measure AFP is offered if there is a family history of neural tube defects, or if earlier screening tests of AFP in the mother's blood indicate that the pregnancy is at increased risk.

Procedure

Ultrasound is used to see exactly where the fetus and placenta are located, which allows the doctor to choose the safest spot for inserting the amniocentesis needle. Then the abdomen is cleansed and sometimes a local anesthetic is injected just beneath the skin. A thin needle is then inserted through the abdomen and into the uterus, where a few teaspoons of amniotic fluid are withdrawn.

After the sample is taken, an ultrasound check of the baby's heart movements is done before the woman leaves the examination room. A few women feel cramping when the needle enters the uterus or pressure during the few minutes while the fluid is being withdrawn. Many doctors recommend that women take it easy for several hours after amniocentesis, avoiding physical stresses such as lifting and prolonged standing.

Living cells from the woman's fluid sample are grown in the laboratory, and then tested for chromosomal abnormalities or various genetic birth defects. Results of laboratory studies of cells after amniocentesis usually take from one to three weeks.

Amniocentesis is usually done at about the 15th week after conception. Now, however, improved ultrasound and laboratory methods have made it possible to consider doing amniocentesis earlier in pregnancy.

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. Amniocentesis test results are more than 99 percent accurate in diagnosing chromosomal abnormalities.

Whether or not to have prenatal diagnosis is a matter for individual discussion between parents and health professionals. Genetic counselors, physicians, and informed religious and ethical counselors can be valuable in helping parents make decisions about prenatal diagnosis and other reproductive issues.

CVS

Tens of thousands of women have had CVS and gone on to deliver healthy babies, but recently several reports have raised new questions about the safety of this test. These reports, which are unproven, have suggested that the procedure may increase the risk of certain birth defects.

Prenatal diagnosis by amnio or CVS is not routinely offered to all pregnant women. Because both tests carry a small risk of infection or miscarriage, doctors offer them only to women who are at increased risk of having a baby with genetic defects or certain major malformations.

These tests are usually offered to women who will be 35 or older when they deliver, since the risk of bearing children with certain chromosomal birth defects increases as a woman ages. The most common of these disorders is Down syndrome, a combination of mental and physical abnormalities caused by the presence of an extra chromosome.

Regardless of age, a woman may be offered CVS or amnio if she already has a child with a chromosomal abnormality, with any of a wide variety of genetic birth defects, or with certain malformations of the spine or brain (such as spina bifida). Similarly, if there is a family history of these disorders, a woman may be offered prenatal diagnosis.

Procedure

CVS is usually performed between nine and 12 weeks after the last menstrual period. At nine to 12 weeks gestation, genetic defects such as Down syndrome can be detected by studying a sample of chorionic villi, the microscopic, finger-like profileions of the developing placenta.

First, the vagina and cervix are cleansed thoroughly with an antiseptic. Then using ultrasound as a guide, the doctor inserts a thin tube through the vagina and cervix to where the chorionic villi are located. Gentle suction is used to remove a small sample of the villi.

Depending on the anatomy, the doctor may use a different method to reach the chorionic villi: inserting a needle through the abdomen, a procedure called transabdominal CVS. A recent study found the two forms of CVS to be equally safe.

After the procedure, spotting is common. Some women find CVS painless, while others experience cramping or feel a pinch when the sample is taken. One in five women has cramping after the procedure; one in three has some bleeding or spotting.

Risk of miscarriage is up to one percent higher than if CVS were not done. Initial results are available within two days. CVS is available only at major medical centers, so many patients have to travel to have the procedure done. A Federal study issued in 1994 reported that CVS increases the risk of an infant being born with missing or underdeveloped fingers or toes. The validity of the reported research has, however, been questioned.

Questions to Ask Your Doctor

Should prenatal testing be done?

What is the risk of having a baby with chromosomal defects?

Why is CVS preferable to amniocentesis in this case?

How will the procedure be done?

Will the procedure (and afterwards) cause discomfort?

Can CVS increase the risk of birth defects?