The Benefits of Amniocentesis and Chorionic Villus Sampling
Tara was a young mom-to-be with two healthy young children. Now, pregnant with her third child, she had the usual pregnancy worries: Was her baby growing well? Would she deliver early?
Then, at 28 weeks along, an ultrasound revealed her baby’s head circumference was too small. The doctors recommended additional testing. After weeks of worry, the test results came back: Her baby had a serious genetic disorder, one so severe that he would likely “never develop language, never walk, never swallow.”
In retrospect, Tara says, there were small signs something might be amiss, but nothing set off alarm bells. Her prenatal genetic screen in her first and second trimester had “come back with one borderline elevated marker.” And no one had recommended additional follow-up testing based her screening results.
Her story is harrowing, heartbreaking, and perhaps enraging, depending on your politics. But there’s one unmentioned fact that merits attention: Had Tara undergone amniocentesis (amnio) or chorionic villus sampling (CVS), she would have discovered about her baby’s genetic condition much earlier, late in her first trimester or early in her second trimester.
As is common for women under 35, Tara only underwent the standard prenatal genetic screening, she never considered diagnostic prenatal genetic tests like amnio or CVS.
Prenatal screenings, the type of prenatal genetic testing doctors tend to recommend for women under 35, miss at least half of the type of genetic problems Tara’s baby carried. Only amnio and CVS can reliably detect them, but women are seldom informed of this important distinction between screens and diagnostic tests.
The type of Genetic screenings doctors most often recommend for women under 35 miss at least half of certain types of genetic problems.
Only Diagnostic tests reliably detect these kinds of Genetic problems.
Prenatal Genetic Screens vs. Tests
Let’s back up for a second. A woman’s options for prenatal genetic testing fall into two basic buckets: screens and tests.
Screens provide a risk estimate. They tell you the likelihood of your baby having a particular genetic disorder, based on indirect markers, like hormones, ultrasound measurements, or, in the case of Non-Invasive Prenatal Screening (NIPT), based on bits of fetal DNA that leak into mom’s bloodstream. But they do not tell you whether your baby has that disorder.
screenings tell you the likelihood of your baby having a particular genetic disorder, but they do not tell you whether your baby has that disorder.
Amnio and CVS, in contrast, are diagnostic tests. They give a clear yes or no answer about whether your baby has a tested-for genetic disorder. There are no false positives, no needless anxiety. And these tests don’t give false negatives (missed diagnoses), such as Tara experienced. If your baby’s test does not find a specific genetic disorder, she does not have that disorder.
Chorionic Villus Sampling and Amniocentesis Cover More Genetic Disorders than Screens
But amnio and CVS offer another, less well known but equally profound benefit: They detect a much wider range of genetic disorders than any of the screens.
Multiple things can go wrong when sperm meets egg. One is that the developing embryo lacks the normal number of chromosomes, leading to disorders like Down Syndrome (caused by an extra chromosome 21) or Turner’s Syndrome (caused by a missing sex chromosome). Another is that a tiny piece of a single chromosome is copied, deleted, or moved from one place to another. And sometimes problematic changes to a just single base pair of DNA, known as point mutations, can occur.
These latter two types, what for convenience we will refer to as small genetic changes, are individually rare. But collectively, they are fairly common. And in terms of consequences for the baby, they can cause no problems, mild problems, or very severe mental and physical problems.
Screens, like Tara had in her first trimester, miss at least half of these small genetic changes. That’s no surprise, as screens were not designed to detect these genetic disorders. They were designed to detect hormonal and ultrasound changes common in three genetic disorders: Down Syndrome, Trisomy 13, and Trisomy 18. When screens do detect other genetic abnormalities, it’s essentially by accident.
In contrast, with recent advances in genetic knowledge and analysis, amnio and CVS can now detect most known small genetic changes, with 100% accuracy.
Amnio and CVS detect a much wider range of genetic disorders than any Prenatal Genetic screen.
For whom does this matter? It matters for all pregnant women.
Once upon the not so distant past, a woman’s doctor decided whether she received prenatal testing or screening, and the choice was made mostly on the basis of her age. Doctors recommended Amniocentesis or Chorionic Villus Sampling for all women ages 35 and older, and screens to younger women.
This age-based approach was simple but not very effective—it missed the majority of chromosomal abnormalities it was intended to detect. Then, as now, the majority of children with Down’s Syndrome and other trisomies are born to younger women, because younger women have more babies, and the screens miss about a quarter of all trisomies.
In 2006, the American College of Obstetricians and Gynecologists updated their guidance on prenatal genetic testing to recommend that amnio and CVs be offered to all women, not just those over 35.
Despite this, doctors often still counsel women under 35 to stick to screens. Insurance companies sometimes refuse to reimburse CVS or amnio for women under 35. And overblown risks of miscarriage from diagnostic testing can scare some women off from these tests.
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But that’s unfortunate. Not only are screens less accurate, they also miss the most common genetic disorders among women under 35. That’s right: Small genetic changes are more common than Down Syndrome, Trisomy 13 and Trisomy 18 in women under 35.
For women under 38, the chances of carrying a fetus with one of these other genetic disorders are roughly 1 in 100—A higher risk than that of carrying a fetus with Down Syndrome, Trisomy 13, or Trisomy 18.
In fact, for women under 38, the chances of carrying a fetus with one of these other genetic disorders are roughly 1 in 100—a higher risk than her chance of carrying a fetus with Down Syndrome, Trisomy 13, or Trisomy 18.
The guidance many pregnant women receive about prenatal genetic testing is based on old technology and old data. It assumes that we can only accurately detect trisomies, not other genetic changes. If we were to recreate the recommendations from scratch, we would not end up here.
The medical system is still playing catch up, and that process may take a decade or more. In the meantime, pregnant women who want clear, yes or no information about the totality of their baby’s genetic health should strongly consider getting Amniocentesis or Chorionic Villus Sampling, whatever their age or family history.
Find everything you ever wanted to know about prenatal testing in our free ebook – The Complete Guide to Prenatal Testing.