Prenatal Testing Options:
There are a multitude of choices for pregnant women regarding testing for certain abnormalities in the fetus. There are TWO MAIN CATEGORIES of tests. The first category are “screening tests” which are noninvasive to the fetus, meaning that there is no increased risk of miscarriage due to the test. However the down side is that screening tests have a high false positive and false negative rate. The second category of tests are “diagnostic tests” and do carry a small increased risk of miscarriage to the fetus. However, the diagnostic tests are very accurate in detecting certain abnormalities.
The first choice for you to make is whether you want to do a screening test OR a diagnostic test. Options are listed for each category below. Often the choices are confusing and a referral to a prenatal diagnostic center for counseling prior to making your final choice can be done.
The physicians in this office believe that if a patient chooses a screening test, that integrated screening has the best detection rates for chromosomal and neural tube defects. However, sometimes your insurance will not cover certain options. You should check with your insurance to determine this.
SCREENING TESTS:
1. Expanded AFP Test or Triple marker test- usually covered by insurance
-this is a blood test on the mother taken between 15-20 weeks gestation which screens for Down Syndrome (mongoloid), trisomy 18 (severe chromosomal defect and baby usually will not survive), neural tube defects like spina bifida ( defect that causes brain defects or spinal cord problems) and another unusual syndrome called SLOS (Smith-Lemli-Opitz Syndrome). It does have a high false positive rate –meaning that the test is abnormal, but the baby is indeed normal. It does not detect all of the abnormalities listed above and is better at detecting neural tube defects than the other abnormalities. It may give information about placental problems that may develop.
2. Combined First Trimester Screening-
This testing is between 11-14 weeks gestation and includes blood tests and a certain ultrasound called a nuchal translucency test. This is the earliest available screening for Down Syndrome and Trisomy 18. It also has a high false positive rate but is lower than for the Expanded AFP test and again does not detect all cases.
3. Integrated Screening –
This is basically screening test #1 AND screening test #2 put together. The test results are a combination of both tests and therefore are available after the triple marker tests return. The false positive rate is lower than each of the individual tests and it picks up more cases of Down Syndrome. However, it does not detect all of the abnormalities it screens for.
4. Second Trimester Ultrasound (fetal survey)-
This is a screening test for major structural abnormalities done generally between 18- 22 weeks. This is not a good screening test for Down Syndrome and cannot guarantee that your baby will be normal. Most of the time your doctor will order this test regardless of the other screening tests you do. A “3D” ultrasound is rarely exclusively used for the fetal survey since it is technically more difficult to obtain the views of the organ systems desired.
After an abnormal screening test, a patient may choose to have a diagnostic test performed to determine if the screening test is a true positive or a false positive.
DIAGNOSTIC TESTS:
1.Chorionic Villus Sampling (CVS)
This test is done in the FIRST trimester (usually 10-13 weeks) to test for chromosomal defects in the fetus (like Down Syndrome). This is a procedure where tissue is taken from the developing placenta. There is a small miscarriage risk from the procedure. It is 98-99% accurate for chromosomal defects. 1-2% of the time, a result is not clear and an amniocentesis is recommended. As this is only a chromosome test, a second trimester ultrasound and the AFP for neural tube defects is recommended in addition to this test.
2. Amniocentesis
This test is done approximately at the time of the second trimester ultrasound at 16 weeks to
20 weeks. This is a test where a needle takes out amniotic fluid around the baby and has a
small miscarriage risk (approximately 1 in 1600). An additional blood test for AFP is not
needed since the information for this test can be obtained from the amniotic fluid.
This is the most accurate (99+%) test for determining a chromosomal
defect in the fetus.