When it comes to prenatal genetic testing, chorionic villus sampling (CVS) and amniocentesis (amnio) are rarely the first types of tests discussed. Most doctors tend to recommend prenatal screenings for women under 35.
But screenings have their shortcoming— they miss at least half of certain types of genetic problems and only provide a likelihood of your baby having a particular genetic disorder. The diagnostic tests (amnio and CVS), in contrast, can reliably detect a range of rare genetic conditions and give a clear answer about whether or not your baby carries a genetic condition.
In addition to leaving these diagnostic tests out of the conversation about genetic testing options, the distinction between screens and diagnostic tests are rarely discussed.
Women deserve better.
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 chorionic villus sampling, 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). If your baby’s diagnostic test does not find a specific genetic disorder, she does not have that disorder.
What is chorionic villus sampling?
CVS tests the genetics of cells in the placental “villi”.
To collect the placental cells, a doctor inserts a needle through the uterus. The position of the placenta determines where the doctor will insert the needle and an ultrasound is used to guide the needle and avoid contact with your baby.
Your doctor will then send the placental cells to a laboratory to be analyzed for common genetic abnormalities and results usually return around a week after extraction.
What is the benefit of chorionic villus sampling vs. amnio?
The main advantage of chorionic villus sampling is that it is performed earlier, between 10 to 13 weeks, allowing for decision making while still in your first trimester. By contrast, amniocentesis cannot safely be performed until 15 to 20 weeks.
Amnio’s main advantage is that in addition to testing for genetic disorders, it also tests for spina bifida, a congenital defect caused by the failure of the neural tube to close completely early in pregnancy (between 21 and 28 days after conception).
Women who undergo CVS will still need a second-trimester alpha-fetoprotein (AFP) test for spina bifida. This AFP test requires only a simple blood draw but is most accurate between 16-18 weeks so it stretches the waiting game into the second trimester. The second-trimester anatomy ultrasound—typically done between 18 and 20 weeks—can also detect some cases of spina bifida.
Chorionic Villus Sampling and Amniocentesis Cover More Genetic Disorders than Screens
A less well known but equally profound benefit of CVS and amnio: 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, 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.
It’s unfortunate that women are scared away from diagnostic tests. Not only are screens less accurate, but 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.