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Prenatal Screening Tests in Second Trimester
Since the mid-1980s, screening has been recommended to detect certain medical and genetic conditions in the fetus between the 16-20th weeks of pregnancy. Researchers found that the blood levels for these special tests were most easily measured at this time in the pregnancy.

Starting with the alpha-fetoprotein (AFP) screening, we have expanded the testing to include three additional blood tests. Together, they are known as the Quad screen. These tests detect 85% of babies suffering from problems with their nervous system (spine and brain), as well as other babies with intestinal problems. They also detect up to 75% of babies afflicted with Down syndrome and other chromosomal problems in women under 35 years old.

These tests are meant to evaluate each woman based upon her age, weight, race, diabetic status, number and gestational age of the baby(ies) she is carrying. When these factors are combined with the blood tests, we are able to predict the likelihood of the baby being affected.

I try to ease my patients' concerns over these blood tests by letting them know their risk of having a child with any of these problems is quite small. Nervous system problems like spina bifida affect only 2 in 1,000 babies and can be reduced markedly if the woman took folic acid (.4mg = 400mcg) before conceiving. And chromosomal problems are based upon the mother's age at the time of the baby's birth. Up until 35, the mother's risk of having a baby with a DNA condition is considered to be rather small.

I have many pregnant patients who will be 35 or older when their baby will be delivered, as well as patients who have a hereditary condition. For these women, the Quad screen is not as accurate. I offer them an amniocentesis or chorionic villus sampling; these procedures are more precise in diagnosing potential problems for these women. Many of these women are very concerned about having a healthy baby. I remind them that the odds of having a healthy baby are still amazingly good. As an example, a 37-year-old pregnant woman has a 1 in 135 chance of having a child with a chromosomal problem. That means that 134 out of 135 times, she will have a healthy baby. Now ask your husband if he'd like those odds. I bet he will.

Nobody likes screening tests because they are like a fishing net cast out into the ocean: we will catch more than the intended target. In this case, we know that between 3-5% of the mothers tested will have an elevated risk. Within this group, we will find the majority of problem pregnancies.

For those with normal test results, congratulations! You have just dodged the biggest obstacle to enjoying the second trimester.

If you have been caught in our screening net and have an elevated AFP or an abnormal Quad screen, let's talk a little more…

Elevated alpha-fetoprotein
If your test shows an abnormally high level, don't panic. There are many reasons for an elevated level such as twins, an incorrect due date or a false positive result. In most cases, your physician will repeat the test before going any further.

While waiting for the second test to come back (the results usually take 3-5 days to return), remember what I said earlier: this is a screening test. Inevitably, there will be some false positive results. And by doing a little math, I can reassure you that if your first test is abnormal, the chances are still 96% of having a normal baby.

A normal result to the repeat test is great news—all is well and so is the baby. We are sorry for your worry and inconvenience. An elevated result should be followed by an ultrasound to look at your baby in more detail. If an explanation is not readily apparent, your obstetrician will discuss further evaluation through an amniocentesis.

Abnormal Quad screen
An abnormal test result indicates a risk of a genetic problem equal to or greater than that of a 35-year-old woman (that's the age at which we routinely suggest an amniocentesis). First, the physician will suggest an ultrasound; this test will verify your due date and look carefully at your baby. If the due date is on target, your doctor will recommend an amniocentesis to check for chromosomal problems. Research studies have found that repeating the Quad screen doesn't improve the detection of a problem. Only an amniocentesis will verify the chromosomal health of the baby.

By now, you are probably wondering if obstetricians spend those long nights in the hospital thinking of ways to torture you with tests. It isn't so. We are trying our best to make sure your baby is as healthy as possible. So when your doctor recommends the Quad screen or amniocentesis, be confident—the odds are overwhelmingly on your side!

As always, if you have any questions, speak with your health care provider.
Craig L. Bissinger M.D.