Amniocentesis is done by your obstetrician in his or her office or in the hospital. An overnight stay in the hospital usually is not needed unless problems occur during the test.

You will be asked to expose your belly. You will then lie on your back with it slightly raised to relax your belly muscles. Your lower belly will be cleaned with a special soap.

Your doctor checks the position of your fetus and the placenta with a fetal ultrasound. Ultrasound uses sound waves to make a picture of the uterus, your fetus, and the placenta on a TV screen. Your fetus's heart rate can also be watched during the test using ultrasound. For more information, see the topic Fetal Ultrasound.

With the ultrasound picture as a guide, your doctor gently puts a thin needle through your belly and into your uterus without hurting your fetus or the placenta. If your fetus moves too close to the needle, the needle will be taken out and your doctor will try again in another spot.

About 2 Tbsp (30 mL) of amniotic fluid is taken out in a syringe attached to the needle, and then the needle is taken out. The site is covered with a bandage.

The whole test takes about 15 minutes. The thin needle is only in your belly for 1 to 2 minutes. Your fetus's heart rate and your blood pressure, pulse, and breathing will be checked before, during, and after the test.