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Department of Obstetrics & Gynecology, University of Washington School of Medicine
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Chorionic Villous Sampling
Genetic Amniocentesis
All pregnant women are now being offered maternal serum screening (also known as AFP testing, double or triple screening) for certain birth defects. It is important that you understand this screening process so you can decide whether or not to be tested. We encourage you to talk with your health care provider about this process.
What
is the purpose of this screening?
Your
unborn baby makes some proteins which can be detected in your blood during
pregnancy. Measuring certain proteins at 15 to 20 weeks from the
first day of the last menstral period can help identify whether you may
be at increased risk to have a baby with a neural tube defect (spina bifida),
Down syndrome, or trisomy 18.
What
is neural tube defect?
A neural
tube defect occurs very early in pregnancy when the cells that become the
skull, brain, and spinal column fail to form a closed tube. Neural
tube defects occur in about one in 1,000 births. Often couples who
have a baby with a neural tube defect have no family history of these problems.
If this tube does not form correctly at the top, the brain and skull will not develop. This is called anencephaly. Babies with anencephaly do not survive.
If there is an opening along the lower portion of the tube, the spine does not close completely. This is called spina bifida. Size and location of the opening determine how severe the spina bifida is. Complications may include problems with walking and bowel and bladder control.
What
is Down Syndrome?
In each
of our cells, we have 46 chromosomes (23 pairs). They give instructions
for our development. Down syndrome results when there is an extra
number 21 chromosome (three, instead of a pair).
Any woman at any age can have a baby with Down syndrome, although the risk increases with the age of the mother. Babies with Down syndrome usually have a mild to moderate degree of mental retardation, as well as other physical problems.
What
is trisomy 18?
Trisomy
18 results when there is an extra number 18 chromosome. Babies with
trisomy 18 have more severe birth defects and mental retardation.
How does
maternal serum screening work?
This
screening measures the levels of up to three different proteins from the
unborn baby or placenta that circulate in your blood:
- Alpha-fetoprotein (AFP)
- Unconjugated estriol (uE3)
- Human chorionic gonadotropin (hCG)
Elevated levels of AFP may mean that the unborn baby has a neural tube defect.
Low levels of all three proteins may mean the unborn baby has trisomy 18.
In pregnancies where the unborn baby is affected with Down syndrome, there may be a special pattern in the levels of all three proteins:
- AFP and uE3 are low
- hCG is high
What
does it mean if my screen is negative?
About
95 out of 100 women screened will have a negative result. If the
Down syndrome risk is found to be less than 1 in 365 (equal to a 35-year-old
womanâs risk) and the AFP level is not elevated, then the result is reported
as ãscreen negative.ä
Unfortunately, a negative screen does not guarantee the baby will be normal, because maternal serum screening does not identify all unborn babies with these problems. It leads to the detection of:
- 6 out of 10 unborn babies with Down syndrome
- 8 out of 10 unborn babies with spina bifida
- 9 out of 10 unborn babies with anencephaly
- 8 out of 10 unborn babies with trisomy 18
What
does it mean if my screen is positive?
In 5
out of 10 of every 100 pregnant women tested, the result is ãscreen positive.ä
This result does not necessarily mean the baby has a problem÷it means that
the unborn baby is at increased risk for a problem.
- At the University of Washington Medical Center, a ãscreen positiveä means:
- The risk of that the unborn baby has Down syndrome is greater than or equal to 1 in 365 (a 35-year-old womanâs risk to have a baby with Down syndrome), OR
- The AFP is more than 2.5 times the normal value, OR
- The risk that the unborn baby has trisomy 18 is increased because all three proteins are low (approximately half the normal values) OR
- Simply that the woman may be 35 or older
Whether or not you have more testing is up to you. You will need to decide if the risk seems high enough to consider diagnostic tests such as ultrasound or amniocentesis.
Remember, The maternal serum screening test is not a diagnostic test. It will not tell you whether or not your unborn baby has or doesnât have a problem. It will simply indicate the chance that the baby has a problem.
Keep in mind that the vast majority of women with ãscreen positiveä results do not have babies with Down syndrome, trisomy 18, or a neural tube defect.
Who
might decide NOT to have screening?
Knowing
about these problems ahead of time may change the management of your pregnancy
and delivery. However, some couples have decided that they would
not consider having an amniocentesis for any reason and/or just do not
want to know during the pregnancy if their baby has one of these problems.
If that is the case, you should not have your blood drawn.
How
confident can I be that the results are accurate?
The University
of Washington Medical Center employs experienced registered medical technologists
to perform these tests.
Our laboratory participates in voluntary quarterly proficiency surveys sponsored by the College of American Pathologists and has a strict quality control program to assure reliable results.
Lab personnel meet frequently with health care providers and genetic counselors to ensure clear communication about laboratory testing and clinical interpretation of test results.
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Chorionic villous sampling (CVS) is a form of prenatal diagnosis that is performed during the first trimester of pregnancy. Chorionic villous sampling provides the earliest diagnosis of problems in fetal development. It can be done between ten and twelve weeks of gestation according to ultrasound dating while the amniocentesis is typically performed after 15 weeks of pregnancy.
The CVS is performed by inserting a thin hollow tube (catheter) through the vagina and the cervix into the uterine cavity. The tip of the catheter is then guided by ultrasound to the tissue that will become the placenta (the villi) and a small amount of tissue is withdrawn. Most women who have had a CVS say the procedure causes little discomfort because the catheter is so thin.
Since part of the placenta, the villi, is of fetal origin these cells reflect the genetic make up of the baby. The villi can be cultured (grown) and tested for many, but not all, genetic disorders. Most commonly, CVS is performed to test for chromosome abnormalities like Down syndrome. When indicated, other biochemical or DNA testing may be available; e.g., Tay-Sachs testing.
Measurement of the alpha-fetoprotein routinely performed on amniotic fluid samples to detect openings in the spine (neural tube defects) is not possible with CVS. Thus, CVS may not be the best option for people with an increased risk to have a baby with spina bifida.
CVS has a higher spontaneous abortion rate than amniocentesis. The national average for miscarriage after CVS is in the range of 1 to 2%.
Results from a CVS take approximately two to three weeks to obtain. In the event of an abnormal result decisions regarding the management of the pregnancy can be made early in the second trimester.
Genetic counseling is provided to fully explain all prenatal testing options, including the benefits and limitations of CVS and amniocentesis. A detailed family history is taken and genetic risks are assessed based on family history, age and other screening factors. Early genetic counseling and risk assessment is desirable to make the most appropriate choices for your pregnancy.
REMINDER: We will need the following information before we can perform a CVS.
- Vaginal cultures for: gonorrhea, chlamydia, Beta-streptococcus, yeast and gardnerella
- Gestational age by ultrasound: the CVS procedure can ONLY be done between 10 and 12 weeks
- Blood type including Rh status
A VERY full bladder is required for a CVS. Please plan on bringing one quart of water to drink within the hour before the CVS appointment.
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Amniocentesis involves the withdrawl of amniotic fluid through a fine needle which is inserted through the motherâs abdomen and into the amniotic sac (bag of waters) surrounding the baby. This fluid contains cells shed from the babyâs skin, lungs and urinary tract. These cells and fluid may then be analyzed for certain genetic problems.
What can prenatal diagnosis tell me about my baby?
1. Chromosomes ö Chromosomes are tiny packets of information that carry the genes that code for our growth and development. There are 46 chromosomes in most normal cells in our body. In the Cytogenetics Laboratory, the babyâs cells from the amniotic fluid are cultured (grown) and the chromosomes in about 20 cells are analyzed. Sometimes, we find either additional or missing chromosomal material that indicates the baby may be born with abnormalities. Chromosome analysis will be able to identify not only Down syndrome (in which there are 47 chromosomes because of an additional number 21 chromosome), but also other types of chromosomal abnormalities in the baby.
2. Neural Tube Defects ö The Department of Laboratory Medicine will measure a chemical known as Alpha-feoprotein (AFP) in the amniotic fluid. This protein, AFP, is normally present in the amniotic fluid in small amounts. However, if there is an opening along the spinal cord or brain (spina bifida or anencephaly), it may leak out in large amounts causing abnormally high levels in the amniotic fluid. Ninety-five percent of open neural tube defects may be detected by amniocentesis. In the general population, the chance of having a child with a neural tube defect is about 1-2 per 1,000. We routinely offer this test because it is an effective screening test that can give you additional information about your baby.
3. Other Information ö In addition to the chromosome analysis and biochemical test for neural tube defects, there are several hundred other conditions that could be performed on the amniotic fluid (such as muscular dystrophy, cystic fibrosis and sickle cell anemia). However, because of the limited amount of fluid and excessive cost of these additional tests, they are not performed routinely on all amniotic fluid samples. This is why it is important for the Genetic Counselor to take a family history and identify any other increased risks prior to having the procedure.
What amniocentesis will not tell us:
It is important to realize that no test can detect all birth defects. There is a 3-5% chance that any pregnancy may end with a child with a birth defect (such as heart defect or cleft lip). Most of these defects are not detectable by amniocentesis. Thus, a normal chromosome test or AFP does not necessarily guarantee a normal baby. However, the reason you were referred to us was because you are at increased risk for having a baby with a problem that could be detected through amniocentesis.
What is the procedure like?
We perform an ultrasound examination before the amniocentesis. You will be asked to lie down on your back and expose your abdomen. (Wear comfortable clothes.) The ultrasonographer will apply some warmed, water-soluable jelly to your abdomen and move a transducer (a microphone-like instrument) around on your abdomen. Sound waves (not X-rays) will bounce off the tissue or fluid-filled spaces in the uterus and a two-dimensional picture will be projected on a screen. Significant features of the baby (i.e. head, spine, heart and limbs) will be pointed out to you if you would like to see them. We use the ultrasound examination for the following reasons:
1. We make sure you are pregnant and there is only one baby present. If more than one baby is seen, we recommend that fluid be taken from around each baby.
2. We correlated the age of the baby with you last menstrual period. A measurement of the babyâs head diameter is a relatively accurate way of determining the babyâs age. We usually perform amniocentesis between 16 and 18 weeks from your last menstrual period. Between 16 and 18 weeks, we have the best chance of obtaining enough fluid while also allowing time to discuss options and make decisions in the event that an abnormality is detected. We will reschedule you for a later date if the ultrasound exam shows you to be less than 14 weeks pregnant. If the ultrasound exam shows you to be over 21 weeks pregnant, you will have to decide whether you still want the test. The results mike come back too late for pregnancy termination to be an option.
3. Ultrasound aids the physician in locating the safest spot to insert the needle. We perform all genetic amniocenteses under ultrasound guidance and actually watch the needle as it is being inserted. In this way, we reduce the chances of touching the baby and increase the success rate of obtaining fluid. It is not unusual to delay the amniocentesis for several days if a good insertion site is not found. This does not mean that anything is wrong with the pregnancy.
Once the Ultrasonographer and Perinatologist have located the safest spot for amniocentesis, a mark is made on your abdomen. The jelly is then wiped off and your abdomen is cleansed with an orange colored antiseptic (betadine). This antiseptic can stain your abdomen or clothes and should be wiped off as much as possible. After this, a thin needle is inserted into your abdomen and into the sac containing the baby. About 2 tablespoons of fluid are withdrawn. Occasionally, patients feel a cramping sensation when this needle touches the uterus.
It is natural to be anxious about the amniocentesis as most of us are not used to having needles inserted into our abdomen. However, almost all women say it is a lot simpler and not as uncomfortable as they expected. The ultrasound examination usually takes about 20 minutes. The amniocentesis usually takes about 2 minutes. You are encouraged to have a support person (i.e. the babyâs father) accompany you during the entire procedure.
What do I need to do before and after the procedure?
There are no special instructions before the procedure. After amniocentesis, we suggest that you avoid heavy lifting and strenuous activity for about 24 hours. You may resume any other normal activities. Immediately after the procedure, you will probably feel more comfortable if someone drives you home.
What are the problems associated with amniocentesis?
Our experience has shown that the risk of amniocentesis is small when performed at our center. There is no evidence that the removal of amniotic fluid in pregnancy has any long term effect on the baby. In fact, the fluid consists mainly of fetal urine which is quickly replenished. Specific risks for problems will be discussed during your counseling session.
Amniocentesis is not an experimental procedure, but a few facts should be emphasized:
1. It is not always possible to obtain amniotic fluid. Occasionally, because of either cramping of the uterus, clumping of cells around the needle, fetal movement, or other factors, we may not obtain fluid and a second needle insertion may be required. It may also be necessary to reschedule your appointment for the following if the physician is unable to obtain fluid after the second attempt.
2. Only a few living cells are obtained in each sample. These must be cultured (or grown) so that we have enough cells for the chromosome analysis. Culturing is why it usually takes between 2 and 3 weeks to get results. Less than 1% of the time, the cells fail to grow within the 2 to 3 week time period. This is not an indication that there is any problem with the baby. It is merely a problem that can happen with any laboratory procedure. If this should happen, we will call you and give you the option of having the procedure repeated.
3. Chromosomal analysis of the fetus is an extremely accurate test with less than 0.5% chance of error.
4. It is not unusual to experience some mild cramping following amniocentesis. This will usually go away by itself. However, any vaginal bleeding, severe abdominal cramps or pain, fainting, fever, or leakage of fluid from the vagina should be immediately reported to your obstetrician and/or the Prenatal Diagnosis Clinic.
When and how will I receive my results?
Results are usually available 2 to 3 weeks after amniocentesis. We will phone you as soon as we receive your results. It is normal to feel anxious while waiting for your tests to come back. Please let us know if you do not want to know the sex of the baby. Your doctor will be informed by mail.
In the event that there is an unusual result, you and the babyâs father will be contacted by phone. We will ask you to come in so that we may explain that we know about the abnormality and discuss your options with you. We will offer you support in your decision and provide you with referral sources if you would like them. Some people choose to terminate a pregnancy based on abnormal finding. Others decide to continue the pregnancy but feel that the advance knowledge helps them prepare both themselves and others for a child who may have special needs. If abnormality is present, we are here to you. Most women have normal results and never have to make these decisions.
IN CONCLUSION
We firmly believe that all women at increased risk to have a baby with a problem should receive Genetic Counseling. But we feel that the decision to have an amniocentesis is a personal matter. You must decide what is best for you by weighing the risks of the procedure against the benefits of obtaining the information. We hope this information has been helpful. Please feel free to contact us if you have any questions.
Last Updated:
3/31/03