If you receive abnormal findings from a pregnancy ultrasound or noninvasive prenatal testing (NIPT), the next step to get definitive genetic information about your baby’s health will likely be one of two prenatal diagnostic tests:
- Chorionic villus sampling (CVS) is typically performed between 11 and 13 weeks of pregnancy and takes a sample from the placenta.
- Amniocentesis, which can be done after week 16 of pregnancy, collects fetal cells from the amniotic fluid around the baby.
Both tests are performed by our maternal-fetal medicine physicians, who specialize in caring for high-risk pregnancies. CVS and amniocentesis can help us determine whether a fetus has a specific chromosomal or genetic condition. Depending on your individual genetic context and family history, we can perform a variety of tests for specific conditions caused by changes in chromosomes or genes.
While prenatal diagnostic testing is optional, the knowledge gained from it can help you prepare for the remainder of the pregnancy and any challenges you and your baby may face.
Understandably, expecting parents often have questions about CVS and amniocentesis and whether they’re effective. Some have also heard myths and misinformation surrounding these very safe tests, so we’d like to answer some of the more common questions as a supplement to conversations with your doctor.
Between 11 and 13 weeks of pregnancy, a maternal-fetal medicine doctor collects cells from the placenta for testing:
- If the placenta is at the front of the mother’s uterus, the doctor will insert a thin needle through the abdomen to remove some cells. This is called the transabdominal method.
- If the placenta is at the back of the uterus, the doctor will instead insert a small tube through the cervix, similar to a pap smear. This is the transvaginal method.
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Often done between weeks 16 to 20, during an “amnio,” the doctor will insert a thin needle through the mother’s abdomen to take a sample of the amniotic fluid that surrounds the fetus. The cells are analyzed in a lab to determine whether the baby has a chromosomal condition.
Amniocentesis can also test for infections or indicators of spina bifida, a birth defect in which the baby’s spinal cord doesn’t fully develop during pregnancy.
Despite what you might have heard, amniocentesis and CVS have a less than 0.2% risk of miscarriage. UTSW specialists follow the most advanced protocols to reduce the risk of test-associated complications. After CVS or amniocentesis, patients are advised to:
- Rest for 24 hours and avoid heavy lifting, strenuous exercise, or standing for long periods of time.
- Refrain from sex for 72 hours.
- Watch for signs of infection such as a fever, or any cramping or vaginal bleeding. Light spotting or cramping can be normal.
No. Both tests last just a few minutes and getting anesthesia would only add more time and an additional needle stick to the process. With both CVS and amniocentesis, some patients might experience what feels like a period cramp that quickly resolves. Most patients are surprised the procedures are so quick and relatively painless.
Yes. Both procedures are as accurate as we can get for chromosomal testing and are considered diagnostic. Neither test routinely checks for every known genetic disorder. However, if there is a family history of a condition caused by a specific gene or parents are known carriers for a specific condition, those genes can also be tested for with diagnostic accuracy.
About 1%-2% of the time, CVS demonstrates a mixture of different types of cells in the placenta, called mosaicism. These can include some normal cells along with some abnormal chromosome cells, making it difficult to know which cells are present in the fetus. If this happens, an amniocentesis is recommended to clarify the result.
The timing depends on which tests are performed and the volume of testing presented to the lab. Some results can come back in a few days, while others may take up to four weeks. Working with your genetic counselor will help you get a clear understanding of which tests will be ordered and what turnaround time you can expect.
Insurance usually accepts the claims if there is a medical indication for testing, which there is for most patients. However, the out-of-pocket cost of the testing will depend on the individual plan and any unmet deductible, co-insurance, or co-pays. You can call your insurance company to ask about coverage for these procedures and prenatal genetic testing.
Even the most accurate tests have some ambiguity, and the testing ordered on a CVS or amniocentesis sample can sometimes detect a “variant of uncertain significance.” This indicates that a genetic anomaly is present but it’s unclear what that variant might mean and whether it could affect the baby’s health. Parental blood tests can sometimes help clarify what the variant might mean. Your genetic counselor will help interpret and explain all genetic testing results from your CVS or amniocentesis and will coordinate additional testing, if needed.
Your genetic counselor will discuss expectations and timetables with you prior to testing. When the results are ready, you’ll speak with your genetic counselor to discuss what the tests did or did not find and start planning next steps.
CVS and amniocentesis are safe, optional procedures that can provide definitive answers about the chromosomal health of a pregnancy. Our goal is to provide information to all parents who need it so they can make informed decisions about the future of their family.