Amniocentesis removes a small amount of fluid from the sac around the baby in the womb (uterus). It is most often done in a doctor's office or medical center. You do not need to stay in the hospital.
You will have a pregnancy ultrasound first. This helps your health care provider see where the baby is in your womb.
Numbing medicine is then rubbed onto part of your belly. Sometimes, the medicine is given through a shot in the skin on the belly area.
The health care provider inserts a long, thin needle through your belly and into the womb. A small amount of fluid (about 4 teaspoons) is removed from the sac surrounding the baby.
The fluid is sent to a laboratory. Testing may include:
Measurement of alpha-fetoprotein (AFP) levels
Results come back in about 2 weeks.
Amniocentesis is also used at times later in pregnancy to diagnose infection, to check to see if the baby’s lungs are developed and ready for delivery, and to remove excess fluid from around the baby when an abnormally high amount of fluid is made (polyhydramnios).
How to prepare for the test
Your bladder must be full for the ultrasound.
Before the test, blood may be taken to find out your blood type and Rh factor. You may get a shot of medicine called RhoGAM if you are Rh negative.
Why the test is performed
Amniocentesis is most often offered to women who are at increased risk for bearing a child with birth defects. This includes women who:
Will be 35 or older when they give birth
Had a screening test result that shows there may be a birth defect or other problem
Have had babies with birth defects in other pregnancies
Have a family history of genetic disorders
You may choose genetic counseling before the procedure. This will allow you to:
Learn about other prenatal tests
Make an informed decision regarding options for prenatal diagnosis
Is a diagnostic test, not a screening test
Is 99% accurate for diagnosing Down syndrome
Is usually done between 14 and 20 weeks
Amniocentesis can be used to diagnose many different gene and chromosome problems in the baby, including:
Simpson JL, Holzgreve W, Driscoll DA. Genetic counseling and genetic screening. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2012:chap 10.
Simpson JL, Richards DA, Otao L, Driscoll DA. Prenatal genetic diagnosis. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, Pa: Elsevier Churchill Livingstone;2012:chap 11.
Susan Storck, MD, FACOG, Chief, Eastside Department of Obstetrics and Gynecology, Group Health Cooperative of Puget Sound, Bellevue, Washington; Clinical Teaching Faculty, Department of Obstetrics and Gynecology, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.