Why it's done

Chorionic villus sampling can provide information about your baby's genetic makeup. Generally, chorionic villus sampling is offered when the test results might have a significant impact on the management of the pregnancy or your desire to continue the pregnancy.

Chorionic villus sampling is usually done between weeks 11 and 14 of pregnancy — earlier than other prenatal diagnostic tests, such as amniocentesis.

You might consider chorionic villus sampling if:

• You had positive results from a prenatal screening test. If the results of a screening test — such as the first trimester screen or prenatal cell-free DNA screening — are positive or worrisome, you might opt for chorionic villus sampling to confirm or rule out a diagnosis.
• You had a chromosomal condition in a previous pregnancy. If a previous pregnancy was affected by Down syndrome or another chromosomal condition, this pregnancy may be at a slightly higher risk, too.
• You're 35 or older. Babies born to women 35 and older have a higher risk of chromosomal conditions, such as Down syndrome.
• You have a family history of a specific genetic condition, or you or your partner is a known carrier of a genetic condition. In addition to identifying Down syndrome, chorionic villus sampling can be used to diagnose many other genetic conditions — including single gene disorders such as Tay-Sachs and cystic fibrosis.

Chorionic villus sampling can't detect certain birth defects, such as neural tube defects. If neural tube defects are a concern, an ultrasound or genetic amniocentesis might be recommended instead.

Your health care provider might caution against transcervical chorionic villus sampling — which is done through the vagina — if you have:

• An active cervical or vaginal infection, such as herpes
• Vaginal bleeding or spotting in the previous two weeks
• An inaccessible placenta, due to a tilted uterus or noncancerous growths in your cervix or the lower part of your uterus

Rarely, your health care provider might caution against transabdominal chorionic villus sampling — which is done through the abdominal wall — if:

• Your uterus is titled backward and your placenta is located at the back of your uterus