What is chorionic villus sampling (CVS)?
Chorionic villus sampling, or CVS, is a prenatal diagnostic test that takes a small sample of tissue from the placenta to check your baby's chromosomes and genetic makeup. The tissue — called chorionic villi — comes from the same fertilized egg as your baby, so its genetic material closely mirrors your baby's own. CVS is a diagnostic test, not a screening test, which means it gives a definitive answer about specific conditions rather than an estimated risk. It's typically offered to people who have had a higher-risk screening result, a family history of a genetic condition, or a personal reason for wanting definitive answers earlier in pregnancy than amniocentesis allows.
CVS is one of two invasive diagnostic tests used in pregnancy, alongside amniocentesis. Both carry a small risk of complications, which is why they aren't offered routinely to everyone — they're offered when the information is likely to be worth that risk. Deciding whether to have CVS is a personal decision, and your provider or a genetic counselor can walk you through what it can and can't tell you before you decide. There's no requirement to have CVS just because it's offered, and choosing not to have it is a reasonable choice too.
When is CVS done during pregnancy?
CVS is typically performed between 10 and 13 weeks of pregnancy, earlier than amniocentesis, which usually happens after 15 weeks. This timing window exists because the placenta needs to be developed enough to sample safely, but sampling earlier than 10 weeks has been linked to a higher chance of limb complications in the baby. Your care team will confirm your gestational age with an ultrasound before scheduling, since timing matters more for CVS than for most other prenatal tests. If you're right at the edge of the window, your provider may recommend waiting a few days or suggest amniocentesis instead.
Because CVS happens earlier than amniocentesis, it can give you results sooner — sometimes with enough time to make decisions or plan additional testing earlier in pregnancy. That earlier timing is one of the main reasons people choose CVS over waiting for amniocentesis. It doesn't suit every situation, though; some conditions, like neural tube defects, aren't detected by CVS at all and need a later test or ultrasound. Your provider can help you weigh the timing advantage against what each test can actually tell you.
How is the CVS procedure performed?
CVS is done in one of two ways — through the cervix (transcervical) or through the abdomen (transabdominal) — and which one your provider uses depends mainly on where your placenta sits. In the transcervical approach, a thin tube is guided through the cervix under ultrasound guidance to collect a small sample of placental tissue. In the transabdominal approach, a thin needle is passed through your abdomen and into the placenta, again guided continuously by ultrasound. Both methods take about 10 minutes, though the ultrasound preparation around it can make the full appointment closer to 30–45 minutes.
Most people describe the procedure as uncomfortable rather than severely painful — a cramping or pressure sensation similar to a strong period cramp, especially with the transcervical method. Local anesthetic can sometimes reduce discomfort with the abdominal approach. You'll usually be advised to rest for the remainder of the day afterward, and mild cramping or spotting in the following day or two isn't unusual. Your provider will give you specific aftercare instructions and let you know what level of discomfort is expected versus what should prompt a call.
What can CVS detect — and what can't it?
CVS can reliably detect chromosomal conditions such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13, along with many single-gene disorders when there's a known family history or risk to test for. Because the sample comes from placental tissue with the same genetic origin as your baby, results for these conditions are considered diagnostic — meaning they give a definitive answer, not a probability. This is different from screening tests like NIPT, which estimate a chance rather than confirm a diagnosis. If a specific genetic condition runs in your family, CVS can often be set up to test for it directly.
What CVS cannot detect: neural tube defects like spina bifida, since these are structural rather than genetic and need a later ultrasound or a blood test (maternal serum alpha-fetoprotein) to assess. It also can't tell you anything about your baby's overall growth, organ structure, or general wellbeing — that's the job of the anatomy scan later in pregnancy. In rare cases, a CVS result reflects a difference between the placenta's cells and the baby's own cells, called confined placental mosaicism, which is why an unclear result is sometimes followed by amniocentesis for confirmation.
What are the risks of CVS?
The main risk parents worry about with CVS is pregnancy loss, and current estimates place that added risk at somewhere around 1 in 300 to 1 in 500 procedures above the background risk of miscarriage at that stage of pregnancy — figures vary by study and by the experience of the center performing it. This is a small but real risk, and it's the central reason CVS isn't offered as a routine test to everyone. Other less common risks include infection, ongoing spotting or bleeding, and, if performed before 10 weeks, a small increased chance of limb abnormalities in the baby.
Call your provider the same day if you experience heavy bleeding, fluid leaking from the vagina, fever, chills, or cramping that gets progressively worse rather than easing over a day or two. Light spotting or mild cramping in the first day or two after the procedure is common and usually not a cause for alarm on its own. Your provider will give you a clear sense, specific to your situation, of what's expected versus what needs prompt attention — ask them directly if anything you're feeling doesn't match what they described.
CVS vs amniocentesis — how do you choose?
The choice between CVS and amniocentesis usually comes down to timing, what you're testing for, and your own comfort with each procedure's risk profile. CVS happens earlier (10–13 weeks) and tests placental tissue; amniocentesis happens later (after 15 weeks) and tests amniotic fluid, which also lets it detect neural tube defects that CVS misses. Both are diagnostic rather than screening tests, and both carry a small procedure-related risk of pregnancy loss, though estimates for amniocentesis are generally slightly lower than for CVS in most studies.
Neither test is 'better' in an absolute sense — they answer overlapping but not identical questions, at different points in pregnancy. Some people choose CVS specifically because it's earlier and gives them more time to plan; others prefer to wait for amniocentesis and its slightly lower loss risk, or need it anyway for the neural tube information. A genetic counselor can talk through your specific risk factors, family history, and what matters most to you, so the choice reflects your situation rather than a general rule.