amniocentesis

Amniocentesis is a way to test a developing baby by sampling the fluid that surrounds it. The amniotic fluid carries cells shed by the fetus, so drawing a small amount gives doctors fetal genetic material to analyze without operating on the fetus itself.

Guided by ultrasound, a thin needle passes through the abdomen into the amniotic sac to withdraw a little fluid, usually in the second trimester. The cells can be examined for chromosomal conditions such as trisomy, and the sample can also be used for targeted gene tests.

It gives a definitive answer where a screening test gave only a probability, but it is invasive and carries a small risk to the pregnancy. Families weigh that risk against the value of certain information, typically with the help of a counselor.