Britannica Sources used in this answer

Why might amniocentesis or chorionic villus sampling follow a positive pregnancy screening result?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

BRCA2 tumor suppressor gene on chromosome 13 — human genetic disease Photomicrograph of the *BRCA2* tumor suppressor gene on chromosome 13 of the human genome. Inactivation of this growth-regulating gene is associated with a higher risk of developing breast cancer.

Amniocentesis or chorionic villus sampling (CVS) may follow a positive pregnancy screening result because screening tests are designed only to assess the likelihood of a condition, not to provide a definitive diagnosis. When screening tests—such as maternal blood tests or ultrasounds—indicate an increased risk for a chromosomal abnormality or other disorder, these invasive diagnostic procedures are performed to confirm the presence of the condition.12

Diagnostic tests are necessary because they allow for the direct collection and analysis of fetal cells or DNA. In amniocentesis, amniotic fluid containing fetal cells is analyzed to determine chromosome morphology or biochemical markers. Similarly, CVS involves sampling placental tissue to identify potential genetic or chromosomal abnormalities. While these procedures are more accurate than screening, they carry a higher risk of complications, including miscarriage, which is why they are typically reserved for cases where a specific risk factor has been identified.132