At a Glance
Why Get Tested?
To detect and diagnose certain birth defects, genetic diseases, and chromosomal abnormalities in a fetus, especially if maternal serum screening tests are abnormal; to evaluate fetal lung maturity when there is an increased risk of premature delivery; sometimes to diagnose a fetal infection; occasionally to help diagnose and monitor hemolytic disease in a fetus
When to Get Tested?
Between 15 and 20 weeks of gestation to test for genetic diseases, chromosomal abnormalities, and open neural tube defects; after 32 weeks to evaluate fetal lung maturity; when it is suspected that a fetus has an infection or other illness; serially, about every 14 days, when it is suspected that a pregnant woman has an Rh or other blood type incompatibility with her fetus
Test Preparation Needed?
You may be instructed to have either a full or empty bladder prior to amniocentesis, depending on when during your pregnancy the testing is being performed; follow any instructions you are given.
The Test Sample
What is being tested?
Amniotic fluid surrounds, protects, and nourishes a growing fetus during pregnancy. Amniotic fluid analysis involves a variety of tests that can be performed to evaluate the health of a fetus.
Amniotic fluid allows a fetus to move relatively freely within the uterus, keeps the umbilical cord from being compressed, and helps maintain a stable temperature. Contained within the amniotic sac, amniotic fluid is normally a clear to pale yellow liquid that contains proteins, nutrients, hormones, and antibodies.
Amniotic fluid begins forming one to two weeks after conception and increases in volume until there is about a quart at 36 weeks of pregnancy. The fluid is absorbed and continually renewed.
The fetus swallows and inhales amniotic fluid and releases urine into it. Cells from various parts of the fetus's body and chemicals produced by the fetus are present in the amniotic fluid. This allows the fluid to be sampled and tested to evaluate fetal health.
The tests are performed on a sample of amniotic fluid that is obtained through a procedure called amniocentesis. Testing may be performed between 15 and 20 weeks of pregnancy to detect certain genetic diseases, chromosomal abnormalities such as Down syndrome, and neural tube defects. Amniotic fluid analysis may be performed at any point after 32 weeks of gestation to evaluate fetal lung maturity when there is an increased risk of or a need for premature delivery. It may also be done when it is suspected that a fetus has an infection or other illness or a blood type incompatibility with the mother and is therefore at risk of developing hemolytic disease.
For genetic testing and chromosome analysis, fetal cells in the amniotic fluid are cultured and grown for 10-12 days in the laboratory, then are analyzed. Biochemical tests, such as bilirubin and alpha-fetoprotein (AFP), and sometimes genetic tests can be performed directly on the amniotic fluid.
The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women should be given the option of having amniocentesis performed. A health practitioner can help a pregnant woman weigh the pros and cons. Some women are at increased risk of birth defects due to their age or family/medical history while others may be advised against having the procedure if they have a history of premature labor, placental problems, or an incompetent cervix, for example. The procedure has some risks associated with it, such as a small chance of miscarriage, and provides information that can have a significant impact of the management of a pregnancy.
While still very much in use, recent advances in testing technology may eventually result in the decline in the use of amniocentesis. For example, there is a newer test called cell-free fetal DNA (cffDNA) that only requires a blood sample from the pregnant woman to screen for certain fetal chromosomal abnormalities, including Down syndrome, Edwards syndrome, and Patau syndrome (trisomy 13), and it can be performed as early as the 10th week of pregnancy. However, at this time, invasive diagnostic tests such as amniocentesis and chorionic villus sampling (CVS) are still needed to confirm the results.
How is the sample collected for testing?
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Depending upon the gestational age of the fetus, either a full or empty bladder may be required at the time the amniocentesis is performed. Be sure to follow any instructions provided.
Ask a Laboratory Scientist
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
March of Dimes. Amniocentesis. Available online at http://www.marchofdimes.com/pregnancy/amniocentesis.aspx through http://www.marchofdimes.com. Accessed Dec 2013.
MayoClinic.com. Amniocentesis. Available online at http://www.mayoclinic.com/health/amniocentesis/MY00155 through http://www.mayoclinic.com. Accessed Dec 2013.
MedlinePlus Medical Encyclopedia. Amniocentesis. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003921.htm. Accessed Dec 2013.
Johns Hopkins Medicine Health Library. Amniocentesis Procedure. Available online through http://www.hopkinsmedicine.org. Accessed Dec 2013.
MedlinePlus Medical Encyclopedia. Hemolytic disease of the newborn. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/001298.htm. Accessed Dec 2013.
UpToDate. Patient information: Amniocentesis (Beyond the Basics). Available online at http://www.uptodate.com/contents/amniocentesis-beyond-the-basics through http://www.uptodate.com. Accessed Dec 2013.
Sources Used in Previous Reviews
(2010 June). Amniotic Fluid Abnormalities. March of Dimes [On-line information]. Available online at http://www.marchofdimes.com/professionals/14332_4536.asp through http://www.marchofdimes.com. Accessed July 2010.
Vorvick, L. (2009 September 2). Amniocentesis. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003921.htm. Accessed July 2010.
Vorvick, L. (2009 September 2). Amniotic Fluid. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/002220.htm. Accessed July 2010.
Singh, D. et. al. (Updated 2010 April 9). Prenatal Diagnosis for Congenital Malformations and Genetic Disorders. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/1200683-overview through http://emedicine.medscape.com. Accessed July 2010.
Hensley, J. et. al. (2010 January 26). A Curious Case of Anti-D Antibody Titer: Fetal Surveillance for RhD Alloimmunization. Medscape from J Midwifery Womens Health. 2009;54(6):497-502 [On-line information]. Available online at http://www.medscape.com/viewarticle/712668 through http://www.medscape.com. Accessed July 2010.
Grenache, D. and Lehman, C. (Reviewed 2009 November). Fetal Lung Maturity - Neonatal Respiratory Distress Syndrome. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/FLM.html?client_ID=LTD# through http://www.arupconsult.com. Accessed July 2010.
Mayo Clinic Staff (2010 May 15). Amniocentesis. MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/amniocentesis/MY00155 through http://www.mayoclinic.com. Accessed July 2010.
Ashwood, E. et. al. (Updated 2010 May). Prenatal Screening and Diagnosis. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/PrenatalScreenDx.html# through http://www.arupconsult.com. Accessed July 2010.
Dungan, J. and Elias, S. (Revised December 2008 ). Prenatal Genetic Counseling and Evaluation, Genetic Evaluation. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merck.com/mmpe/sec18/ch257/ch257b.html#sec18-ch257-ch257b-885 through http://www.merck.com. Accessed July 2010.
Driscoll, D. and Gross, S. (2008 January). First trimester diagnosis and screening for fetal aneuploidy. American College of Medical Genetics Practice Guidelines January 2008 V10(1) [On-line information]. Available online at http://www.acmg.net/AM/Template.cfm?Section=Search2§ion=Practice_Guidelines1&template=/CM/ContentDisplay.cfm&ContentFileID=286 through http://www.acmg.net. Accessed July 2010.
(2008 August). Amniocentesis. March of Dimes Quick Reference: Fact Sheets, [On-line information]. Available online at http://www.marchofdimes.com/professionals/14332_1164.asp through http://www.marchofdimes.com. Accessed July 2010.
Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 54-59, 484.
Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Pp 373-376.
(January 10, 2008) CDC. Toxoplasmosis: Health care providers. Available online at http://www.cdc.gov/toxoplasmosis/hcp.html through http://www.cdc.gov. Accessed October 2010.
Nemours Foundation. Cytomegalovirus, Diagnosis and Treatment. Available online at http://kidshealth.org/parent/infections/bacterial_viral/cytomegalovirus.html#a_Diagnosis_and_Treatment through http://kidshealth.org. Accessed October 2010.
(May 2009) March of Dimes. Cytomegalovirus infection in pregnancy. Available online at http://www.marchofdimes.com/professionals/14332_1195.asp through http://www.marchofdimes.com. Accessed October 2010.
(April 2007) March of Dimes. Fifth Disease in Pregnancy. Available online at http://www.marchofdimes.com/professionals/14332_25586.asp through http://www.marchofdimes.com. Accessed October 2010.