Your Essential Guide to Prenatal Testing: What to Expect When

The home pregnancy test that reveals you are expecting (squee!) is just the first of many tests you’ll be offered to help you learn more about your baby. We consulted leading experts to sort out how the latest screenings work and what the results mean.

Pregnant Woman in Red Off-the-Shoulder Dress Thayer Allyson Gowdy
Toward the end of Ana Paula Mello’s first trimester, her doctor offered her a blood test that would reveal her baby’s sex and screen for genetic conditions. Mello, who lives in Los Angeles, consented. About two weeks later, she got a call from her ob-gyn requesting that she and her husband come in to discuss the results. In the office, they were told that the test indicated her baby had an increased risk of the rare DiGeorge syndrome, which can cause heart defects, cleft palate, developmental delays, and other problems.

Crying, Mello asked if the results could be wrong. The test was highly accurate, her doctor said, and she scheduled an appointment for Mello and her husband to see a genetic counselor that day.

While the ob-gyn had made the disorder sound almost certain, the counselor explained that the test was just a screening, so the results were not definitive. Only an invasive procedure, like amniocentesis, which tests genetic material present in the baby’s amniotic fluid, or chorionic villus sampling (CVS), which analyzes cells from the placenta, could confirm a diagnosis. Both come with an extremely small risk of miscarriage. Mello opted for the CVS test two days later.

This time the results brought good news. “Everything was fine, and I was so relieved,” says Mello, who went on to deliver a healthy boy. However, the terror she felt before speaking to the genetic counselor still haunts her. “I thought I was just finding out whether I was having a boy or a girl,” she says. “I never expected I might get negative news.”

Mello’s experience is not that uncommon. The number of prenatal genetic testing options has exploded, confounding patients and doctors. Many couples are agreeing to tests without knowing how they work or what they might uncover, says Katie Stoll, a genetic counselor with the Genetic Support Foundation in Olympia, Washington. “To make informed decisions, you have to understand each test’s potential benefits, risks, and limitations.” 

How Testing Has Evolved

A few decades ago, the only genetic screening test was this question: How old are you? If you were 35 or older, you’d automatically be offered an amniocentesis. Today, women are presented with a lot of screening options that are noninvasive, so they pose no risk to you or Baby, but the results are not definitive. Like Mello, you could get a false positive. Or you could get a false negative. Although these tests are optional, most women choose basic screening to find out if their baby might have any serious problems that could be helped in utero or would benefit from special care at birth, says Mary E. Norton, M.D., a professor of obstetrics, gynecology, and reproductive sciences at the University of California, San Francisco. 

Which Test When?

There are two common types of noninvasive genetic tests: carrier screenings and prenatal screens.

A carrier screening is a blood test that both you and your partner can receive to learn if one or both of you carry the genes for an inheritable condition, like cystic fibrosis, that can be passed down from mother and father to child. You can get a carrier screening before conception or anytime during pregnancy. Many of the conditions these screenings look for are autosomal recessive, meaning both partners need to be carriers to pass down the condition. Some are more prominent in specific populations, such as sickle cell disease in people of African origin and Tay-Sachs disease in Eastern European Jews.

There are now expanded carrier screenings that can check for hundreds of heritable conditions for just a few hundred dollars, but the American Congress of Obstetricians and Gynecologists (ACOG) only recommends carrier screenings based on your ethnic background or family history of disease. Even if you and your partner are advised to get screened, and you both test positive for the same recessive condition, that doesn’t mean your child will definitely be affected. Depending on the condition, your doctor will refer you to a genetic counselor to discuss your next steps.

Prenatal screens, the second type of noninvasive genetic tests, calculate the risk of chromosomal issues that are the result of random errors in cell division. The most common of these conditions is Down syndrome, which affects about 1 in 700 pregnancies.

“Noninvasive prenatal screenings are by far the most common prenatal tests, and the most complicated,” says Jennifer Hoskovec, a boardcertified genetic counselor at the University of Texas Health Science Center at Houston. All of them require bloodwork and some an ultrasound. “The goal is not to find out definitively whether or not there’s an abnormality with your baby, but to better define how likely an abnormality might be,” she says.

One of the most widely offered options is the first trimester screen, done between 11 and 13 weeks of pregnancy. It factors in your age, the levels of some hormones and proteins in your blood, and an ultrasound measurement of fluid at the back of your baby’s neck—known as a nuchal translucency scan—to reach a risk score for Down syndrome and the more serious, often fatal Trisomy 18. This number is usually delivered as a probability, such as a 1 in 2,000 chance of having a baby with Down syndrome. About 5 percent of women will get a high-risk result, meaning an increased risk for a chromosomal condition. The screening detects between 80 to 85 percent of Down syndrome pregnancies, but it also results in false positives.

Pregnant Woman in Red Off-the-Shoulder Dress with Husband Thayer Allyson Gowdy
Another common noninvasive screen is the quad screen, typically done between weeks 15 and 22. This test looks at four substances in your blood that, depending on their levels, can signal an increased risk for Down syndrome, neural tube defects, or other conditions. Used alone, it identifies about 80 percent of Down syndrome pregnancies, but given in conjunction with the first trimester screen, it identifies up to 93 percent. Known as integrated or sequential screening, combining both tests provides a more accurate result and fewer false positives.

While these two screenings have been offered as part of prenatal care for more than a decade, a third option, cell-free DNA screening (cfDNA), has emerged in the last five years. Known under their brand names, such as MaterniT21 PLUS, Harmony, verifi, and Panorama, they gather fragments of fetal DNA floating in your blood for analysis and can be given anytime after ten weeks of pregnancy. They’re marketed as more advanced and accurate than the traditional tests, but “patients need to understand that cell-free DNA screenings are not perfect and can result in false positives and false negatives,” says Joseph R. Biggio Jr., M.D., chair of ACOG’s genetics committee. In fact, this latest generation of screenings has come under fire for several reasons. See “Buyer Beware” (below) to learn more.

If any of these screens reveal your baby may be at high risk for a genetic condition, the invasive diagnostic tests CVS or amniocentesis, both of which come with a small risk of miscarriage, will provide a definitive answer. CVS, which is what Mello chose, is performed between ten and 13 weeks and involves removing a bit of the placenta for testing, either through your abdominal wall or with a catheter passed through your cervix. Mello says it didn’t hurt but that she did have some muscle soreness for a few days afterward. Other women have described the procedure as a more uncomfortable Pap smear. Because this test does not scan for neural tube defects, such as spina bifida, a second trimester screen is often also done later to provide a risk estimate for those conditions.

Amniocentesis, which has been around for decades and can be performed after 14 weeks, involves inserting a long, thin needle through your abdomen and into the amniotic sac to remove a few tablespoons of fluid for testing. It’s painless and relatively fast. Your doctor may spend more time pinpointing exactly where to place the needle, using ultrasound as a guide, than actually drawing the sample. With both tests, it can take two to three weeks to get results because your baby’s cells need to be grown in a lab before they can be analyzed.

Understand Your Results

The good news is that false negatives are uncommon. That means that if a screening reveals your baby has no chromosomal abnormalities, you can feel pretty confident about that result. However, false positives can be common, especially in women over 35, notes Dr. Norton, and they can lead to stress and anxiety.

If you do receive a positive screening result for a condition, the rarer the condition is, the greater the likelihood that it’s wrong. The risk of having a child with DiGeorge syndrome (the condition Mello was told might affect her baby) is about 1 in 4,000 births, so the chance that a positive screen will actually result in a baby with DiGeorge syndrome is pretty low, says Dr. Norton.

When you’re in the moment, navigating all of this may feel overwhelming, which is why if you’ve had a positive screening result, it’s crucial to consult a prenatal genetic counselor to help you understand the nuances of testing. Confirm that the counselor is certified by either the National Society of Genetic Counselors or the American Board of Genetic Counseling.

As anxiety-ridden as the testing process can be, most women feel that knowledge is power. “If I were to get pregnant again, I would definitely do the same thing,” Mello insists. “I would have the screening first, and if something came up, I would do the CVS again. It was scary, but it’s better to know and be prepared.” 

Buyer Beware for Some Prenatal Tests

You may have heard about cell-free DNA tests (cfDNA). While there were only four labs that offered them in 2014, today there are at least eight, plus more than a dozen brands of tests that screen for Down syndrome, Trisomy 18, Trisomy 13, as well as rarer conditions like DiGeorge syndrome. But experts say cfDNA tests should only be used for high-risk pregnancies because research shows more than half of the positive results are wrong in low-risk pregnancies. “These are for-profit tests being offered by for-profit companies,” says Megan Allyse, Ph.D., a biomedical ethicist at the Mayo Clinic in Rochester, Minnesota. Because the FDA doesn’t regulate these genetic tests, companies don’t have to publish how well their tests work, she says. Experts caution women not to make decisions about terminating their pregnancy based on these screenings without getting a diagnostic test first.

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