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.