Prenatal Invasive Tests

Invasive tests are used to extract fetal matter for the purpose of a genetic exploration of the fetal DNA. The tests look at the chromosomal arrangement to rule out chromosomal problems such as Down syndrome.
There are three invasive tests that can be done at different stages of the pregnancy:

• Chorionic villus sampling (CVS): In this test a part of the placenta is sampled during the first trimester (weeks 10-13). This test presents a 1% risk of miscarriage.
• Amniocentesis: In this test some of the amniotic fluid is extracted as of week 17. This test presents a 0.5% risk of miscarriage. The alpha-fetoprotein is also checked during this test, because high levels of this protein can indicate a defect in the neural tube.
• Blood sampling from the umbilical cord: In this test a blood sample is extracted from the umbilical cord of the fetus.
  There are other diseases that can be identified with the help of CVS or amniocentesis, using molecular or biochemical methods. These diseases are checked when there's suspicion that the fetus has a specific disease and in cases when a genetic mutation was located in the family.

Before performing an invasive test, the woman receives genetic counseling.

Women who meet one or more of these criteria are eligible to undergo these tests as part of the health care basket:

• At least 35 years old at the beginning of the pregnancy
• Increased risk for Down syndrome based on first or second trimester screening tests
• High alpha-fetoprotein levels or low estriol levels
• Anomalous findings during an ultrasound
• Chromosomal problems during previous pregnancy
• One of the parents was identified as a carrier of a mutation that causes a genetic disease (such as cystic fibrosis) or a chromosomal translocation
• The woman contracted a virus that presents a risk to the fetus (such as SMV or toxoplasma)

Any invasive prenatal diagnosis requires a genetic counseling session beforehand!

The tissue received through CVS or amniocentesis can be analyzed using a chromosomal microarray analysis (CMA), a method used to identify tiny chromosomal changes in the fetus. This technology allows for the identification of hundreds of diseases and syndromes that cause mental retardation and other problems. The recommendation for a CMA analysis is usually given when there are anomalous findings in the ultrasound. At Shaare Zedek Medical Center the CMA analysis is offered without extra charge to any woman undergoing an invasive test.

Another available analysis method is QF-PCR, a quick test that checks just five chromosomes (13, 18, 21, X, Y). The results are received within three work days. This method requires additional payment.

On the day of the test please be sure to bring with you your pregnancy follow-up card or a document that states your blood type, along with the results of all the tests performed so far. It is highly recommended that you bring the results of your genetic screening (whether done for this pregnancy or for a previous pregnancy). The couple will receive genetic counseling that includes a thorough explanation of the procedure and the woman will be asked to fill out forms and sign a consent form. This counseling is only in regards to the procedure and doesn't replace counseling that deals with abnormal findings.

The procedure will be done by a gynecologist under an ultrasound scan.
Results are received within 2-4 weeks. The result is given by a genetic counselor over the phone, and if further exploration is needed, the couple is invited for a follow-up counseling session.