Here's what you should expect from screening and diagnostic tests—and what the results will tell you.
What Prenatal Tests Are Required?
During the course of my prenatal visits (I'm nine months pregnant and due any day now), I was tested for everything from cystic fibrosis to Down syndrome. Whenever I went in to see my ob-gyn, it seemed to be time for another test. I thought these tests—used to check for chromosomal abnormalities—were mandatory, but I've since learned that your doctor is only required to offer them. You don't have to accept.
Depending on your beliefs, you might not even want to have every test. "The real question is, what are you going to do with the information you get?" says Lorien Tambini, genetic counseling supervisor at Roosevelt Hospital, in New York City. "If there's a problem, do you terminate the pregnancy or continue the pregnancy and keep the baby? It's a very personal decision."
Here's an overview of some of the common tests you should expect to get during your pregnancy. If your nerves get the better of you and you start to worry, remember that only 5 percent of all pregnant women receive an atypical test result during pregnancy, and because of false positives and erroneous results, even fewer women will have a child born with a genetic or physical abnormality.
How Are the Tests Conducted?
There are four primary methods your doctor can use to determine whether everything is okay: ultrasound, blood tests, amniocentesis, and chorionic villus sampling. The first two options are screening tests, which means the results indicate whether your baby could have a health issue. The second two, on the other hand, are diagnostic tests. They're more invasive—both analyze a fetus's DNA—but they're also more conclusive. A diagnostic test will tell you for sure if your baby has a condition you should know about.
What Are the Initial Prenatal Tests?
All pregnant women receive a battery of tests during their first prenatal visit. The list includes blood tests—to check for chlamydia, gonorrhea, hepatitis B, syphilis, cystic fibrosis, Rh factor, and HIV—as well as a urine culture and a Pap smear. You and your doctor should discuss any concerns you have at this visit, or whenever they come up along the way, and ask lots of questions. The more you know, the less anxious you'll feel.
What Do the Tests Determine?
Technology is now so advanced your doctor can use noninvasive methods, such as ultrasounds and blood tests, to determine if your baby is at risk for Down syndrome or other chromosomal abnormalities. The first-trimester screening, which is performed between 11 and 14 weeks, combines a nuchal translucency (NT) ultrasound with a blood test to determine the likelihood that your baby will have a chromosomal abnormality. The NT measures the amount of fluid at the back of the fetus's neck. Extra fluid—indicated by a thicker neck—could mean a higher risk for Down syndrome.
The second-trimester screening, also known as the triple or quadruple screen, is performed between 15 and 18 weeks. The triple and quadruple screens are blood tests that check for certain substances (either three or four of them, hence the names) that might indicate Down syndrome. The results of the screen are entered into a computer, along with your age, to calculate the risk of your child having the condition. (The results do not apply to any future children you might plan to have.) Both the triple and the quadruple screens include a test for alpha-fetoprotein (AFP), which checks for neural tube (spinal cord) defects.
Some ob-gyns offer an "integrated screening." This test combines the data from the first-trimester screening with the results of the triple or quadruple screen (the second-trimester screening). Some doctors believe this may be the most accurate method for diagnosing Down syndrome.
It's important to understand that these tests don't give a positive or a negative result. Instead, you get a probability; you might learn, for example, that there's a 1 in 100 chance that your baby will have Down syndrome. "Some patients don't realize that these screening tests don't give you definitive information," says Margaret Buxton, a certified nurse-midwife at the West End Women's Health Center, in Nashville. However, if your numbers translate to your being low-risk, the doctor will probably tell you everything is fine.
What Are the Diagnostic Tests?
Your doctor will probably offer you an amniocentesis if one or more of your screening tests yield a high-risk result. If you're age 35 or older, which alone makes you high-risk, it's likely you'll be given the option of getting the amnio from the start and skipping the screenings. During an amniocentesis, which can be done as early as 15 weeks, a physician inserts a needle into your abdomen to draw a sample of amniotic fluid from the womb. This test can identify about 40 fetal abnormalities and determine conclusively if your baby has Down syndrome, cystic fibrosis, or sickle cell disease.
There is a risk, though, with the test—about a 1 in 200 chance that you will lose your baby. But this rate varies, and some studies suggest that risk of miscarriage after amnio is lower with more experienced doctors.
Chorionic villus sampling (CVS), in which your doctor places an instrument through the cervix or a needle through the abdomen to remove a small piece of tissue from the placenta, is an alternative to amniocentesis. CVS checks for abnormal levels of hemoglobin and enzyme deficiency and measures your placental tissue for Down syndrome, cystic fibrosis, sickle cell disease, hemophilia, Tay-Sachs disease, and thalassemia. The upside of CVS is that it can be done earlier in pregnancy (between 10 and 12 weeks). But the test isn't available at all clinics, and it may carry a higher risk of miscarriage than amnio.
How Should You Address Your Concerns?
Before you have any of these tests, it's important to discuss all of them—and their consequences—with a medical professional and your significant other. "The tests we're talking about are optional," Tambini says. "No one is ever going to tell a pregnant woman, 'you have to do this.' Everything is always up to the patient." If your test results reveal an abnormality, your doctor and/or genetic counselor will help you understand what the results mean and what your options are going forward.
What Is Genetic Counseling?
A genetic counselor is a medical professional who can talk with you and your partner about your chances of having a child with a genetic disorder or birth defect.
Your doctor may refer you to a genetic counselor if you're over 35, if you receive abnormal results from a screening test, or if you have other risk factors, such as a family history of a heritable disease. Depending on your test results, a genetic counselor can help you understand and navigate your options.
Though the idea of getting genetic counseling may not sound reassuring to you at first, many couples find that the information helps them better understand their options without causing any extra worry.
Used with permission from American Baby magazine.