and Chorionic Villus Sampling

Most women in the United States give birth to healthy babies.  Despite this, many women worry about birth defects.  Some women are at higher risk than others for having a baby with a birth defect.  While not all birth defects can be found before delivery, certain tests can help to find some birth defects during pregnancy.  Two such tests are amniocentesis and chorionic villus sampling (CVS).

 

ABOUT BIRTH DEFECTS

     About 3% of babies born in the United States have some type of major birth defect.  These birth defects can affect the way an infant looks or how the baby's body and organs-including the brain-function.  A baby may be born with more than one birth defect.  In some cases, there is no known reason for a birth defect.  In some cases, birth defects are inherited (passed by parents to their children).  others can occur if the fetus is exposed o certain drugs (including alcohol), chemicals, or viruses during the key stages of growth during pregnancy.  In some cases, a doctor may find that a birth defect is random and that a family does not have increased risk for another child with problems. 

 

GENETIC PROBLEMS

     Genes are passed by parent to their newborn through tiny structures called chromosomes.  Chromosomes are present in every human cell.  the mother's egg and the father's sperm each contain 23 chromosomes.  Each chromosome carries many genes.  These genes determine growth and development, sex, and the traits (such as hair and eye color) that the child will have.  When the sperm joins the egg, the genes and chromosomes of both parents unite to form those of the fetus.  Genes also control how the cells of the body work and determine the growth of a fetus from the fertilized egg.  Thus, an error in a chromosome or  a single gene may result in birth defects.  A birth defect can be passed from the parents to their baby through their genes.  This is called a genetic disorder.  A birth defect can occur even when neither parent has the disorder or a family history of birth defects.  A defect also may result from problems with the number or structure of whole chromosomes.  There may be too many or too few chromosomes.  In Down syndrome, for instance, there is an extra (third) copy of one of the chromosomes.   Some genetic problems can be found in two ways:

1. •Amniocentesis-studying the amniotic fluid that surrounds the fetus.
   

2. •CVS-studying the chorionic villi that make up the placenta.
   

     For these procedures, a sample is taken to be tested from either the amniotic fluid that surrounds the fetus or the placenta.  Amniocentesis and CVS can help find chromosomal problems such as Down syndrome.  They also may detect other genetic diseases such as cystic fibrosis, Tay-Sachs disease, and sickle cell disease.  These tests can be used to find out the sex of the fetus.  Knowing the sex of the fetus can help find disorders that are linked to one sex.  Certain types of muscular dystrophy, for instance, occur most often in males. 

 

WHO SHOULD BE TESTED?

     Your doctor can tell you about your genetic risks and the tests you can have.  Only you and your partner can decide whether to have a test.  You should learn about the tests and any risks involved.  How far along your pregnancy is and whether the tests can be done in your area are two points to think about.  Testing should be offered to:

1. •Pregnant women who will be 35 or older on their due date (the risk of having an infant Down syndrome increases with the age of the woman)
   

2. •Couples who already have had a child with a birth defect or have a family history of certain birth defects
   

Pregnant women with other abnormal genetic test results.

     A normal test result on the fetus cannot ensure that the baby will be normal.  This is for a number of reasons.  First, each test performed on the fetus looks for a certain problem.  There may be a problem that the test was not designed or able to find.  Second, some problems cannot be detected by testing.  Finally, no test is 100% foolproof. 

 

AMNIOCENTESIS

     Amniocentesis is the most common procedure used to test for birth defects.  It is done at 16-18 weeks of pregnancy in most cases.  it is done in a hospital or in the doctor's office.  You don't need to stay overnight. 

 

THE PROCEDURE

     With amniocentesis, a sample of amniotic fluid is withdrawn through a needle from the sac that surrounds the fetus.  Amniotic fluid contains cells from the fetus that can be tested.  These cells have the same genetic makeup as the fetus.  For the procedure, the patient lies down with her abdomen exposed.  First, ultrasound is used to show the doctor where to insert the needle to try to avoid touching the fetus.  The needle is then guided through the abdomen and the uterus into the amniotic sac.  S small sample (about 1 ounce) of fluid is withdrawn.  If you are carrying twins, the doctor will need to take a sample from each sac.  The fetus will produce more amniotic fluid to replace the fluid that is withdrawn.  The amniotic fluid is sent to the lab.  The cells are grown in a special fluid for several days.  Then, tests are done.  The tests that are done depend on your own and your family's history.  Rarely, the cells don't grow.  Then, the procedure may need to be done again.  This does not mean that the fetus has a problem. 

 

RESULTS

    It may take about 2 weeks for enough cells to grow and tests to be performed.  The chromosomes within the cells are studied under a microscope.  The number and shape of the chromosomes are checked.  Also, special tests for specific genetic diseases can be done on the cells.  Tests of the amniotic fluid itself are another way to find some defects.  One such test is the alpha-fetoprotein (AFP) test.  AFP is a protein made by a growing fetus.  Small amounts of AFP are passed into the amniotic fluid.  Too much AFP in the amniotic fluid can be  a sign of fetal defects, such as open neural tube defects or opening in the fetal abdomen.  One type of AFP is a blood test.  It checks the levels of AFP in the woman's blood.  This test may help a woman decide whether to have amniocentesis. 

 

RISKS

    Although amniocentesis is fairly safe, there is some risk involved.  Side effects that may occur include:

1. •Cramping
   

2. •Bleeding
   

3. •Infection
   

4. •Leaking of amniotic fluid after the procedure
   

5. •Miscarriage
   

     Injury to the fetus from amniocentesis is rare.  All pregnancies have some chance of ending in a miscarriage-whether a test is done or not.  very early in pregnancy, the risk of natural miscarriage is higher.  Later in pregnancy, it is lower.  The normal risk of miscarriage at the time when amniocentesis would be done is 2-3%.  The risk of miscarriage is increased very slightly with amniocentesis-less than 1 in 200 women who have the test will have a miscarriage that they would not have had otherwise. 

 

CHORIONIC VILLUS SAMPLING

     With CVS, a small sample of cells is taken from the placenta where it is attached to the wall of the uterus.  Chorionic villi are tiny parts of the placenta.  Villi are formed from the fertilized egg.  Therefore, they have the same genes as the fetus.  There are two ways to collect cells from the placenta: through the vagina or through the abdomen.  To collect cells through the vagina, a speculum is inserted just as for a Pap test.  Then, a very thin, plastic tube is inserted up the vagina and into the cervix.  With ultrasound, the tube is guided up to the placenta.  A small sample is removed.  To collect cells through the abdomen, a slender needle is inserted through the woman's abdomen to the placenta, much like amniocentesis.  The sample of chorionic villi then is sent to a lab.  There the cells are grown and tested.  If you have an active sexually transmitted disease (STD), bleeding during pregnancy, or certain problems with the cervix, you may be offered CVS through the abdomen.  In other cases, neither type may be a good choice. 

 

RESULTS

     CVS can detect most of the same defects as amniocentesis.  One defect that cannot be detected by CVS is open neural tube defects.  If you have CVS, you may want to think about having a blood AFP test later in the pregnancy to screen for neural tube defects.  The results of CVS can be obtained earlier in pregnancy and more quickly than with amniocentesis.  Most women get their results within 10 days. 

 

RISKS

     CVS may carry a slightly higher risk of miscarriage than amniocentesis.  The rate is higher than that for amniocentesis because CVS is done earlier.  Infection also can occur.  Limb defects in infants may occur, especially if CVS is done before 10 weeks.  This is rare though.

 

ADVANTAGES OF EACH METHOD

     Amniocentesis and CVS both have advantages.  With amniocentesis:

1. •It often is easier to have done because it is more widely available.
   

2. •It offers a lower risk of miscarriage than CVS.
   

3. •It can test for neural tube defect because it test the amniotic fluid.
   

     On the other hand, CVS can be done earlier in pregnancy.  Getting this kind of information early allows a woman to make choices in the early stage of her pregnancy.  If a woman chooses abortion because she receives abnormal results, the abortion will be safer than if she waits for amniocentesis results. 

 

OPTIONS

     Most of the time, tests show normal results, which reduce patients' fears and anxieties.  If you tests diagnose a major birth defect, you have tough choices to make.  You may choose to continue the pregnancy and have the baby.  or you may choose to have an abortion.  If you choose to have they baby, you may need to deliver at a special hospital.  You also may need extra help after the baby is born.  If you choose to have an abortion, you should decide as soon as possible.  The earlier an abortion is done, the safer it is for you.  Before you decide, get as much information about the defect as you can-from doctors, counselors, parents of a child with the same type of defect.  Ask friends or family for advice and support.  Knowing as much as you can will help you to make the best choice. 

 

FINALLY...

     If you have certain risk factors, you may be offered amniocentesis or CVS to try to detect certain birth defects.  Whether you have the test done is up to you.  Some people choose not to get this information.  Keep in mind that in most pregnancies, babies are born healthy.