What to expect when you're expecting (27 page)

What is it?
Chorionic villus sampling (CVS) is a prenatal diagnostic test that involves taking a small tissue sample from the finger-like projections of the placenta called the chorionic villi and testing the sample to detect chromosomal abnormalities. At present, CVS is used to detect disorders such as Down syndrome, Tay-Sachs, sickle cell anemia, and most types of cystic fibrosis. CVS cannot test for neural tube and other anatomical defects. Testing for specific diseases (other than Down syndrome) is usually done only when there is a family history of the disease or the parents are known to be carriers. It is believed that CVS will eventually be able to detect over 1,000 disorders for which defective genes or chromosomes are responsible.

How is it done?
CVS is most often performed in a hospital, though it can also be performed in a doctor’s office. Depending on the location of the placenta, the sample of cells is taken via the vagina and cervix (transcervical CVS) or via a needle inserted in the abdominal wall (transabdominal CVS). Neither method is entirely pain free; the discomfort can range from very mild to moderate. Some women experience cramping (similar to to menstrual cramps) when the sample is taken. Both methods take about 30 minutes, start to finish, though the actual withdrawal of cells takes no more than a minute or two.

It’s a … Surprise!

Diagnostic testing can determine your baby’s gender. But unless it’s a necessary part of the diagnosis, you’ll have the option of either learning the results when you receive the CVS or amnio results (if you haven’t already found out through ultrasound results) or waiting to find out the old-fashioned way, in the birthing room. Just make sure your practitioner knows about your decision ahead of time so your surprise won’t be inadvertently spoiled.

In the transcervical procedure, while you lie on your back, a long thin tube is inserted through your vagina into your uterus. Guided by ultrasound imaging, the doctor positions the tube between the uterine lining and the chorion, the fetal membrane that will eventually form the fetal side of the placenta. A sample of the chorionic villi is then snipped or suctioned off for diagnostic study.

In the transabdominal procedure, you’ll also lie tummy up. Ultrasound is used to determine the location of the placenta and to view the uterine walls. Then, while still under ultrasound guidance, a needle is inserted through your abdomen and the uterine wall to the edge of the placenta, and the cells to be studied are drawn up through the needle.

Because the chorionic villi are of fetal origin, examining them can give a clear picture of the genetic makeup of the developing fetus. Test results are available in one to two weeks.

When is it done?
CVS is performed between the 10th and 13th weeks of pregnancy. Its main advantage is the fact that it can be performed in the first trimester and it can give results (and most often, reassurance) earlier in pregnancy than amniocentesis, which is usually performed after the 16th week. The earlier diagnosis is particularly helpful for those who might consider a therapeutic pregnancy termination if something is seriously wrong, since an earlier abortion is less complicated and traumatic.

How accurate is it?
CVS is able to accurately detect chromosomal abnormalities 98 percent of the time.

How safe is it?
CVS is safe and reliable, carrying a miscarriage rate of about 1 in 370. Choosing a testing center with a good safety record and waiting until right after your 10th week can reduce any risks associated with the procedure.

Some vaginal bleeding can occur after CVS and should not be a cause for concern, though it should be reported. You should also let your doctor know if the bleeding lasts for three days or longer. Since there is a very slight risk of infection with CVS, report any fever that occurs in the first few days following the procedure.

What is it?
Like the first-trimester combined screening, the integrated screening test involves both an ultrasound and blood test results, but in this case, the ultrasound (to measure NT) and a first blood test (to measure the concentration of PAPP-A) are performed during the first trimester, and a second blood test (to measure the same four markers in the blood as in the quad screen. see below) is performed during the second trimester. All three tests are integrated to give you the results. Like other screening tests, the integrated test doesn’t directly test for chromosomal problems, nor does it diagnose a specific condition; rather, the results merely provide you with your baby’s statistical likelihood of having a problem. Once you have that information, you can decide, together with your practitioner, whether you want to undergo diagnostic testing.

When is it done?
The ultrasound is performed between the 10th and 14th weeks. The first blood test is done on the same day as the ultrasound, and the second blood test is done between 16 and 18 weeks. The results are reported to you after the second blood test.

How accurate is it?
A test gathering and integrating information from both the first and second trimester of pregnancy is more effective than one that uses information from the first or second trimesters alone. With the integrated screening test, about 90 percent of Down syndrome cases and 80 to 85 percent of neural tube defects are detected.

How safe is it?
Both the ultrasound and the blood tests are painless and carry no risk to the mom or her baby.

What is it?
Quad screening is a blood test that measures the levels of four substances produced by the fetus and passed into the mother’s bloodstream: alpha-fetoprotein (AFP), hCG, estriol, and inhibin-A. (Some doctors test only for three of the substances in a triple screen.) High levels of AFP may suggest the possibility (but by no means the probability) that a baby is at higher risk for a neural tube defect. Low levels of AFP and abnormal levels of the other markers may indicate that the developing baby may be at higher risk for a chromosomal abnormality, such as Down syndrome. The quad screening, like all screening tests, can’t diagnose a birth defect; it can only indicate a higher risk. Any abnormal result simply means that further testing is needed.

Interestingly, studies indicate that women who receive abnormal results on their quad screen but receive normal results on follow-up testing such as amniocentesis
may
still be at very slightly increased risk of certain pregnancy complications, such as a small-for- gestational-age fetus, preterm delivery,
or preeclampsia. If you receive results like this, ask your practitioner what steps you can take to reduce the chances of any potential complications later on, keeping in mind that the association between abnormal results and such complictions is very small to begin with.

When is it done?
Quad screening is performed between weeks 14 and 22.

How accurate is it?
The quad screen can detect an increased risk of the defects for approximately 85 percent of neural tube defects, nearly 8. percent of Down syndrome cases, and 80 percent of trisomy 18 problems. The false-positive rate for the independent quad screen is high. Only 1 or 2 out of 50 women with abnormally high readings eventually prove to have an affected fetus. In the other 48 or 49, further testing reveals that the reason the hormone levels are abnormal is that there is more than one fetus, the fetus is either a few weeks older or younger than originally thought, or the results of the test were just wrong. If the woman is carrying only one fetus and the ultrasound shows the dates are correct, an amniocentesis is offered as follow-up.

How safe is it?
Because the quad screen requires only a blood sample, it is completely safe. The major risk of the test is that a positive result may lead to follow-up procedures that present greater risk. Before you consider taking any action on the basis of prenatal screening, be sure an experienced physician or genetic counselor has evaluated the results.

What is it?
The fetal cells, chemicals, and microorganisms in the amniotic fluid surrounding the fetus provide a wide range of information about the growing baby inside you, such as genetic makeup, present condition, and level of maturity. Being able to extract and examine some of the fluid through amniocentesis has been one of the most important advances in prenatal diagnosis. It is offered when:

Results of a screening test (the combined first-trimester screen, the integrated screen, triple or quad screen, or ultrasound. turn out to be abnormal, and evaluation of the amniotic fluid is necessary to determine whether or not there actually is a fetal abnormality.

The mother is older (usually over age 35), primarily to determine if the fetus has Down syndrome (though an older mom with reassuring screening results may opt, in consultation with her practitioner, to skip amnio).

The couple has already had a child with a chromosomal abnormality, such as Down syndrome, a metabolic disorder, or an enzyme deficiency such as cystic fibrosis (CF).

The mother is a carrier of an X-linked genetic disorder, such as hemophilia (which she has a 50 percent chance of passing on to any son she bears).

Both parents are carriers of an autosomal recessive inherited disorder, such as Tay-Sachs disease or sickle cell anemia, and thus have a 1 in 4 chance of bearing an affected child.