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Why it's done
By Mayo Clinic staffChorionic villus sampling can provide information about your baby's genetic makeup. Generally, chorionic villus sampling is offered when the test results may have a significant impact on the management of the pregnancy — or your desire to continue the pregnancy. Often, the value of this information can only be assessed by you and your partner.
Chorionic villus sampling is usually done between the 10th and 12th weeks of pregnancy — earlier than other prenatal diagnostic tests, such as amniocentesis.
You may consider chorionic villus sampling if:
- You had abnormal results from a prenatal screening test. If the results of a screening test — such as the first trimester screen — are positive or worrisome, you may opt for chorionic villus sampling to confirm or rule out a diagnosis.
- You had a chromosomal abnormality in a previous pregnancy. If a previous pregnancy was affected by Down syndrome or another chromosomal abnormality, this pregnancy is at higher risk, too.
- You're age 35 or older. Babies born to women age 35 and older have a higher risk of chromosomal abnormalities, such as Down syndrome.
- You have a family history of a specific genetic disorder, or you or your partner is a known carrier of a genetic disorder. In addition to identifying Down syndrome, chorionic villus sampling can be used to diagnose many other genetic disorders — such as Tay-Sachs disease and cystic fibrosis. This requires specialized testing of the chorionic villi, however, so there must be a reason to test for these conditions.
Chorionic villus sampling cannot detect neural tube defects, such as spina bifida. If neural tube defects are a concern, an ultrasound or genetic amniocentesis may be recommended instead.
