At a Glance
Why Get Tested?
To detect and diagnose some birth defects, genetic diseases, and chromosome abnormalities in a fetus, especially if pregnancy screening tests are abnormal; to evaluate fetal lung maturity; 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, chromosome abnormalities, and open neural tube defects; when there is an increased risk of premature delivery, after 32 weeks to evaluate fetal lung maturity; when it is suspected that a fetus has hemolytic disease, about every 14 days
Test Preparation Needed?
You may be instructed to have a full or empty bladder prior to amniocentesis.
The Test Sample
What is being tested?
Amniotic fluid surrounds, protects, and nourishes a growing fetus during pregnancy. It allows the baby to move relatively freely, 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 pregnanacy. 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.
Amniotic fluid analysis encompasses a range of tests that can be performed to evaluate the health of a fetus. The tests are performed on a sample of amniotic fluid that is obtained through a procedure called amniocentesis. These tests may be performed on samples obtained between 15 and 20 weeks of pregnancy to test for some genetic diseases, chromosome abnormalities such as Down syndrome, and neural tube defects. It 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 a blood type incompatibility with the mother and is at risk for developing hemolytic disease.
For genetic testing and chromosome analysis, fetal cells in the amniotic fluid are cultured and grown for several days in the laboratory, then are analyzed. Biochemical tests, such as bilirubin and alpha-fetoprotein, and sometimes genetic tests can be performed directly on the amniotic fluid.
How is the sample collected for testing?
A sample of amniotic fluid is obtained using a procedure called amniocentesis
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 that you are given.
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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.
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