How To Understand What Prenatal Genetic Test Results Actually Mean

The testing available to expectant parents has changed rapidly in recent years, giving us more information at earlier stages of pregnancy. But understanding your test results can be tricky.
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The nuchal translucency screening is performed via ultrasound.

Conception is often compared to a roll of the dice. You hope for a particular outcome but know that your success depends a great deal on luck. Every step of the way, you face odds: your odds of conceiving at your age, your odds of having a miscarriage, your odds of developing a pregnancy complication. 

The majority of pregnancies are low risk, but it can be hard not to think about scary possibilities during each stage of pregnancy. Most of us are eager to hear that our baby will be born healthy. Prenatal genetic testing often provides families with this sort of reassurance, but sometimes the results of these tests can bring more uncertainty. There have been rapid advances in prenatal testing in recent years, but more information at earlier stages of pregnancy can lead to misunderstanding and confusion if we aren’t able to correctly interpret test results. 

It’s important to note that each person’s risk is unique to them, based on age, ultrasound findings and other factors, and should be discussed with a health care provider. It can be helpful, however, to have some background information about what these tests can and cannot tell us in preparation for receiving your results and talking with your doctor, midwife or genetic counselor. 

What prenatal genetic tests are available?

Screening tests commonly offered include the cell-free fetal DNA test (a blood test) and the nuchal translucency screening (a measurement taken during an ultrasound). The diagnostic tests available are chorionic villus sampling (CVS — cells taken from the placenta) and amniocentesis (in which a sample of amniotic fluid is taken). 

What types of conditions can these tests detect?

Genetic tests look for certain kinds of irregularities in genes. They are commonly used to detect trisomies, where there are three copies of a gene instead of the normal two. By far the most common trisomy is Trisomy 21, or Down syndrome, occurring in 1 in 700 births. 

Other common trisomies that tests screen for are Trisomy 13 and Trisomy 18, also known as Patau syndrome and Edwards syndrome, respectively. Trisomy 13 occurs in approximately 1 in 6,000 births, and Trisomy 18 occurs in about 1 in 3,000.

Holly Mueller, a genetic counselor at the University of California, San Francisco, told HuffPost that when a fetus has trisomy 13 or 18, “it’s a very life-limiting condition. Many pregnancies that have Trisomy 13 or Trisomy 18 will not survive the pregnancy. Oftentimes, if those babies are born, they will not survive for very long after birth,” although there have been exceptional cases of children living past one year.  

By arranging chromosomes into pairs, scientists can also identify the sex of your baby (note that knowing the sex of your baby is not the same as knowing their gender identity). Females have two X chromosomes and males have one X and one Y — but there are exceptions. 

If you opt to find out the sex of your baby, tests will also determine if there is a trisomy, an extra sex chromosome, or a monosomy, only one sex chromosome rather than two. A fetus with monosomy X (only one X chromosome) has Turner syndrome, which can cause infertility and heart defects. Sex chromosome trisomies are also a possibility. XXX syndrome, in a female, often has no symptoms at all. Klinefelter syndrome, or XXY, results in a male fetus that, when born, may go on to have infertility or other symptoms; it is often not diagnosed until adulthood. 

What is the difference between a screening test and a diagnostic test?

One common point of confusion is the difference between a screening test and a diagnostic one. A screening test will tell you the odds that your baby has a specific condition, not whether they actually have it. A positive result on a screening test does not mean that your baby has a genetic disorder — only a diagnostic test can determine that. 

Dr. Neeta Vora, professor of obstetrics and gynecology and director of reproductive genetics at the University of North Carolina at Chapel Hill, told HuffPost that she explains it this way to her patients: “A screening test gives you a result that tells you either you have a low chance or a high chance of having a certain condition. A diagnostic test tells you a yes-or-no answer. So if you get a high chance on a screening test, then we’re going to recommend that you have a diagnostic test.” 

The purpose of screening tests is to select who is more likely to have a condition and offer diagnostic tests — which are more invasive — to those people rather than having everybody take the diagnostic test.

What does it mean if you get a positive result on a screening test? 

To further complicate matters, the odds calculated for you with a screening test are more accurate for certain conditions, and in certain cases, than others. This is what’s called the positive predictive value.

“The chance that you get a positive result that’s a true positive is the positive predictive value,” Vora said. A true positive would mean you got a positive screening test that was then confirmed by a diagnostic test. A false positive, on the other hand, would mean you got a positive screening test but then a diagnostic test found that your baby did not have the condition.

The positive predictive value “is based on the prevalence of the disease in the population you’re screening,” Vora said.

She gave the example of a Down syndrome diagnosis, a risk we know increases with age. “If you’re 20 years old, the prevalence of you having a baby with Down syndrome is much lower than if you’re 40. So if you get a positive screening test for Trisomy 21, the positive predictive value is going to be much lower ... compared to someone who’s 40.” For the 20-year-old in this scenario, the positive predictive value would range from 38% to 80%, whereas for a 40-year-old, it would range from 91% to 99%. 

In either case, only a diagnostic test could determine whether the screening test was a true positive or a false one. 

In addition to testing for trisomies and monosomies, there are some companies that offer tests for microdeletions, which are missing bits of DNA. However, according to a New York Times investigation published in January 2022, these tests produced more false positives than true ones — meaning the positive predictive value for these much rarer conditions was low. If you wanted to look at the whole genome in this way, you would have to opt for CVS or amniocentesis for a more accurate result.

What’s the difference between the test options?

Cell-free fetal DNA

Screening or diagnostic? Screening.

Procedure: A blood sample is taken from the pregnant person. “DNA that’s been released from a cell in the placenta is circulating in the maternal plasma,” explained Vora. This DNA is examined. 

What it can detect: Cell-free fetal DNA tests are generally used to find trisomies 21, 18 and 13. They can also test for sex and abnormalities in the number of sex chromosomes, although you do not have to have this test run. Some companies offer tests for microdeletions, although, as described above, the positive predictive value for these conditions is low. The American College of Obstetricians and Gynecologists does not recommend cell-free fetal DNA testing to screen for microdeletions. 

Pros: This test can be performed early, at 10 or even nine weeks. Because it is a non-invasive test and requires only a blood draw, there is no risk to the pregnancy. Many people are excited to run the test because it will reveal the baby’s sex so early. The test is very accurate, more accurate than the screening tests previously available. 

Cons: Because it is a screening test, you will need a diagnostic test to confirm that your baby indeed has one of these conditions. There is also the possibility of a false negative — meaning the test is negative but your baby does have the condition. Some people choose to proceed with a diagnostic test even when their screening test is negative.

If you choose to test for your baby’s sex, you may also screen positive for a sex chromosome abnormality. “That’s kind of the downside,” Vora said. “If you screen positive for one of those just wanting to know the gender, then you’re in this spiral of anxiety” as you await diagnostic testing. 

In very rare cases, the test will indicate an abnormality in the pregnant person’s chromosomes or even the presence of cancer in their body. 

Nuchal translucency

Screening or diagnostic? Screening.

Procedure: At 10 to 13 weeks of pregnancy, an ultrasound technician takes a measurement of a fluid-filled space at the back of the baby’s neck. 

What it can detect: Abnormal measurements of this space at the back of the neck can indicate Down syndrome or other trisomies, and are also associated with other birth defects (of the heart, stomach or skeleton). These abnormalities would be found on another ultrasound later in the pregnancy. 

Pros: Ultrasound is a non-invasive test that poses no risk to the pregnancy. Vora explained that if a nuchal translucency screening is positive, she would recommend that a patient skip cell-free fetal DNA testing and go straight to diagnostic testing. 

Cons: A positive screen does not mean that there is definitely a problem, only that further testing is recommended. Nuchal translucency screening is less accurate at detecting Down syndrome than the cell-free fetal DNA test but is still used by some doctors. 

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JGI/Jamie Grill via Getty Images
Prenatal genetic screening tests can be performed as early as 9 to 10 weeks into the pregnancy.

CVS (chorionic villus sampling) 

Screening or diagnostic? Diagnostic. 

Procedure: Performed at 11 to 13 weeks of pregnancy, sometimes up to 14, “CVS is a sample of the placenta. It can either be done abdominally, with insertion of a needle through the abdomen, or transcervically, with insertion of a catheter through the vaginal canal to take a sample of the placenta,” Mueller explained. 

What it can detect: In addition to Down syndrome, trisomies 13 and 18, and differences in the number of sex chromosomes, a DNA sample taken from the placenta can be checked for other genetic disorders with microarray testing, which tests for microdeletions or duplications in all of the chromosomes. 

Pros: CVS is extremely accurate and can be performed earlier than an amniocentesis. 

Cons: There is a small risk of miscarriage, which Vora estimated to be 1 in 500. There is also a small chance, 1% to 2%, that the test results show placental mosaicism — meaning the some of the cells in the placenta are normal and some are not. In these cases, patients would need to have another test, amniocentesis, to confirm a diagnosis. 


Screening or diagnostic? Diagnostic. 

Procedure: Amniocentesis is usually performed between 15 and 20 weeks of pregnancy. Guided by ultrasound, a doctor uses a long needle to take a sample of amniotic fluid from the uterus. This fluid contains cells from the baby, which can then be examined. 

What it can detect: All of the same conditions as CVS. 

Pros: Amniocentesis is very accurate, even in cases of placental mosaicism, and can be used to identify many conditions. 

Cons: There is a small chance of miscarriage, which Vora estimated to be 1 in 1,000. Amniocentesis is performed later in pregnancy than CVS and cell-free fetal DNA testing. 

What should you do if you receive a diagnosis?

Some patients decline prenatal genetic testing, reasoning that they would not choose to terminate a pregnancy regardless of the results. But there are other ways that genetic testing can help families. Knowing your child’s diagnosis before birth can speed their entry into care for their condition. It also gives you time to do research and talk to other families that have been affected by this condition so you can prepare for what’s ahead. 

Finally, keep in mind that you can’t know how you will react to something before it happens. Some people think that they know for sure how they would proceed if they received a diagnosis of a genetic disorder but when faced with that reality they feel differently, or more uncertain, than they had imagined. A genetic counselor can help you understand what a specific condition might mean so that you can make the choice that is best for you and your family. 

For patients considering pregnancy termination, access to abortion has been drastically restricted since the fall of Roe v. Wade last year. Abortion is now effectively banned in 12 states. Other states have set in place bans at 15, 18, 20 or 22 weeks of pregnancy, shrinking the window of time in which a person who has received the results of a diagnostic test would be able to access abortion — and in some cases eliminating that possibility. If you live in a state where abortion access is restricted and you need abortion care, you can call the National Abortion Hotline or find information about abortion funds from the National Network of Abortion Funds

If you’re having trouble understanding your test results, a genetic counselor can help. Your midwife or OB-GYN may be able to find one for you, or the National Society of Genetic Counselors has a database, which you can search by location. It also provides contact information for counselors you can see online, using telehealth connections. 

“As long as families kind of have the information to make the choice that’s right for them, that’s the most important thing ― less so having one path that’s perfect for everybody,” Mueller said. 

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