Amniocentesis for genetic diagnosis is most often performed between fifteen and twenty weeks of pregnancy, though it can be performed later if needed, including in the third trimester for reasons such as a new finding on a later ultrasound. Testing this early or later than the usual window is generally avoided when possible, since data suggest it carries a somewhat higher chance of complications and, particularly before fourteen weeks, laboratory difficulty growing enough cells for testing.

The procedure itself is done in an outpatient setting and typically takes only a few minutes:

Ultrasound guidance is used throughout. Before the needle is inserted, ultrasound is used to check the baby’s position, the location of the placenta, and a safe pocket of fluid to sample, and ultrasound continues to guide the procedure in real time.

A thin needle is passed through the abdomen. The doctor cleans the skin over the abdomen and, guided by the ultrasound image, gently inserts a thin, hollow needle through the abdominal wall and the wall of the uterus into the amniotic fluid.

A small amount of fluid is withdrawn. About one to two tablespoons of amniotic fluid is drawn out through the needle. The body replaces this fluid naturally within about a day.

The needle is removed and the baby’s heartbeat is checked. After the sample is collected, the needle is withdrawn, and the baby’s heartbeat is typically checked by ultrasound before the patient goes home.

Most patients describe the sensation as similar to a blood draw or a brief cramp rather than significant pain, though a numbing medication for the skin can be used if desired. Afterward, mild cramping or spotting can occur and usually settles on its own; patients are generally advised to rest and avoid strenuous activity for the remainder of the day and to contact their care team promptly if they notice fluid leakage, heavy bleeding, fever, or severe cramping.