What Moms-to-Be Should Know About Prenatal Genetic Testing

June 13, 2018 7:00 AM

Many parents choose to know if their unborn child might have birth defects. A Michigan Medicine genetic counselor explains the tests that can inform them.

When a woman learns she’s pregnant, a barrage of appointments and information comes quickly. This includes early decisions about prenatal genetic testing.

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The optional tests, which can help detect the risk of abnormalities, can supply important information before a baby’s birth.

“The amount of genetic information we can get about the pregnancy from a simple blood draw is expanding rapidly,” says Michigan Medicine Von Voigtlander Women’s Hospital genetic counselor Beth Dugan, M.S., C.G.C.

Although the results can help families make informed decisions, the tests aren’t perfect. Blood-based screening tests cannot say with certainty that a baby is affected, a deficit that can be difficult to handle during pregnancy.

Which is why it’s a good idea to talk about any type of testing before a baby is conceived, Dugan says.

“Some of those discussions are easier to have outside the context of pregnancy,” she says. “I think most couples would want to talk about what they would do if the results are positive and if they’re on the same page.”

Also key: weighing each test’s capacities, risks and limitations.

Dugan spoke about the methods of prenatal testing and how genetic counselors play a role:

Pregnancy screening options

Women choose to undergo or decline screening for a variety of reasons. There are two blood-based methods to screen for chromosome abnormalities in pregnancy:

First-trimester screening: This test has two parts: bloodwork looking at hormones in a mother’s blood and an ultrasound. The ultrasound is used to measure the back of the baby’s neck (known as a nuchal translucency scan); extra fluid on the neck can be a sign of Down’s syndrome, heart defects or other complications.

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This information is used to provide an overall risk assessment. The test will detect about 93 percent of Down’s syndrome cases with a 5 percent false positive rate. It also screens for other chromosome abnormalities and can sometimes detect pregnancies at high risk for complications such as poor growth later in pregnancy.

Cell-free DNA screening: This test also requires a blood draw from the mom. DNA fragments from the placenta that are in a pregnant mother’s blood can be used to try to identify pregnancies at higher risk for chromosome abnormalities. “The assumption is the placenta DNA is going to match the baby’s,” Dugan says.

This test is the most sensitive screening for Down’s syndrome, trisomy 13 and trisomy 18 (genetic disorders that cause birth defects) and provides some information about the baby’s sex chromosomes. It has a lower false positive rate than the first-trimester screen but does not provide other information about a baby’s health.

Each screening may be administered as early as 10 weeks into a pregnancy. That said, “we don’t recommend you do both,” notes Dugan.

Diagnostic testing in pregnancy

Diagnostic tests provide more accuracy as well as information about conditions not included in screening.

“We move beyond common chromosomal problems and have the ability to scan for smaller deletions or duplications,” Dugan says. This can be particularly informative when certain birth defects are detected on an ultrasound.

Some women may choose to have these invasive tests without screening. Others, though, could have an abnormal screen and then proceed to a diagnostic test.

Diagnostic testing options include:

Chorionic villus sampling: Placenta tissue is extracted via the cervix (a similar approach to a Pap smear) or by using a needle inserted through the mother’s abdomen into the uterus. It is performed between the 10th and 13th weeks of pregnancy. Parents who want a confirmation sooner often choose this option, Dugan says.

Amniocentesis: Here, a needle is inserted through the abdomen to extract fluid surrounding the baby (the needle doesn’t touch the baby). The amniotic fluid, which Dugan says “is mostly baby urine,” contains fetal cells that can be used for diagnostic testing. It is performed 15 weeks into the pregnancy or later.

There is a risk of miscarriage with these procedures; most women who undergo the procedures have no complications.

“You’ll hear different risks quoted, but we try to stay pretty conservative,” says Dugan, noting that the miscarriage risk quoted by Michigan Medicine’s Maternal Fetal Medicine is 0.28 percent for chorionic villus sampling and 0.25 percent for amniocentesis.

Blood-based screenings, by comparison, pose no risk to a fetus.

How genetic testing affects parents

The majority of patients who choose to have prenatal genetic testing do so for a simple reason: knowledge.

But what they do next, particularly if the test results are concerning, is where professional guidance is crucial in both the long and short term.

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Says Dugan: “It can lead you down a very different road when the results come back.”

At Michigan Medicine, patients with abnormal screening results are referred to a genetic counselor such as Dugan. These individuals have psychosocial training and are prepared to help parents not only interpret the results but also process the emotional part of the news.

“Parents are thrown into a situation that is obviously anxiety-provoking,” Dugan says. “Even when they get abnormal results, some tell us that they’re glad they have that information to plan or prepare.”

Talk with your prenatal care provider about your screening options or contact the Fetal Diagnosis and Treatment Center at C.S. Mott Children’s Hospital and Von Voigtlander Women’s Hospital at 734-763-4264.