PRENATAL GENETIC SCREENING

Why are these tests offered?
For many years, those involved in prenatal care have attempted to prenatally diagnose the more common severe newborn problems. Down's Syndrome represents the most common chromosomal (genetic) disease that occurs. Since the problem occurs more frequently as a woman becomes older, prenatal testing like amniocentesis is offered to women over 35. However, only about 25% of Down's syndrome children are actually born to women over 35. (Many more women under 35 actually give birth). Prenatal genetic screening was developed to try to identify women under 35 who can be shown to have a high enough risk to offer them prenatal genetic testing like amniocentesis.

Can I do screening tests if I'm over 35?
These tests were originally designed for women under 35, assuming that women over 35 would just go ahead and do the amniocentesis that is offered. There are a number of women over 35 however who would elect to forego more definitive testing if a screening test significantly lowers their risk to an acceptable level. We do perform these screening tests on those women but give the following precautions:

Screening tests can never completely rule out the presence of Down's Syndrome, but only reduce the likelihood of occurrence. If you would do the amniocentesis regardless of the screening test findings, you should not do the screening test.

There are other, much less common chromosomal abnormalities that screening tests may not reveal that also increase in likelihood with increased maternal age. Amniocentesis would show these as well.

Different Tests Available:
For many years patients have been offered the "triple-screen" test, which includes three tests run on a sample of mother's blood: alpha feto-protein, hCG, and estriol. This test is currently the standard test for most practices.

Elite Diagnostics offers two additional methods of screening: the Nuchal Translucency ultrasound test, and the Ultrascreen Test, which combines the nuchal translucency with two tests run on a fingerstick sample of the mother's blood. These last two tests offer two advantages:

They are run earlier in the pregnancy. The triple screen is run between 15 and 20 weeks, whereas the NT and Ultrascreen tests are run between 11 and 13 weeks. Earlier screening leads to more options for further testing.

They are more sensitive tests. When talking about sensitivity, it's easiest to think of it in terms of "pick-up rate."

The "Pick-Up Rate" for Down's Syndrome:
If all pregnancies were screened with the triple screen, about 65% of Down's Syndrome would be picked up.

If the Nuchal Translucency ultrasound test was performed, that number would increase to about 80%.

If the full Ultrascreen test is done, the pick-up rate increases to over 90%.

Remember that only 25% percent of Down's Syndrome are picked up if testing is done based on age alone. Some patients believe that second trimester ultrasound can identify Down's Syndrome, but this is not reliable. Unfortunately, only about 50% of Down's Syndrome fetuses will show a suspicious finding.

How to Decide about Prenatal Genetic Screening:
The various prenatal screening tests will not tell you if anything is definitely wrong, do not diagnose Down's syndrome or other defects, but rather determine your risk for carrying these birth defects.

The risks quoted as high by geneticists or the laboratory may be not seem high to you. Down's syndrome is considered to be high risk by most labs if the risk is higher than 1 in 250.

Since prenatal screening tests only determine your risk, their real intention is to identify women who should be offered amniocentesis, or in the case of early tests, possibly chorionic villus sampling. Offered does not mean you must do this test, but without follow-up testing, these screening tests have little value.

Amniocentesis and CVS carry risks, about 1 in 200 risk of losing the pregnancy in the case of amniocentesis, and probably slightly higher in the case of CVS. These risks are due to the unavoidable risk of introducing infections into the amniotic cavity, usually resulting in miscarriage.

Before you do a screening test, know what you will do with an abnormal test. How high a risk is high enough to warrant amniocentesis for you? Would a 1 in 100, or 1 in 50 risk of Down's syndrome be high enough for you to want to do an amniocentesis? Also, you should consider what you would do with an abnormal result on amniocentesis? Would you terminate a pregnancy complicated by Down's Syndrome?

Although knowing early that the fetus has Down's has value whether termination is an option for you or not, only you can decide whether this value outweighs the risks.

How to Decide about Amniocentesis or CVS:
Try to picture worse case scenarios.

On one hand, if you don't do the test, you might deliver a baby with Down's Syndrome that you could have previously diagnosed.

On the other hand, you could do the test, but lose the pregnancy as a results of this choice.

Which "bad outcome" would be harder for you to deal with? Disregarding the numbers confusion, concentrating on the small but unavoidable possibility of having to deal with one of these possible outcomes is probably a better way to decide.

In our office we counsel patients that there are two types of patients who should not do prenatal screening:

1. Patients who are offered testing (amniocentesis) due to age or other risks and would do an amniocentesis regardless what the screening test shows.

2. Low risk patients who would never do amniocentesis no matter what the screening test shows.

Remember, screening tests primarily are to identify candidates for amniocentesis. If the screening test would not alter your decision to do or not to do an amniocentesis, prenatal genetic screening tests may not be for you.

Check out the decision page to help you decide about prenatal screening, as well as a breakdown of the various tests available, including their advantages and disadvantages.

Click here to go to a detailed technical description of the various prenatal screening tests.