To detect and diagnose certain birth defects, genetic diseases, and chromosomal abnormalities in a fetus, especially if prenatal screening tests are abnormal; if there is a family history or high risk of inherited disease; sometimes to diagnose a fetal infection and occasionally to help diagnose and monitor hemolytic disease in a fetus; rarely, to evaluate fetal lung maturity when there is an increased risk of premature delivery
Testing is most commonly offered between 15 and 20 weeks of pregnancy to test for genetic diseases, chromosomal abnormalities, and open neural tube defects and is recommended after a pregnant woman has an abnormality on an initial prenatal screening test; when it is suspected that a fetus has an infection or other illness or when a fetus has hemolytic disease; rarely it may be done after 32 weeks to evaluate fetal lung maturity
A sample (about 1 ounce) of amniotic fluid is obtained using a procedure called amniocentesis that involves inserting a thin needle through the belly and uterus into the amniotic sac (see Common Questions below).
Depending upon the stage of pregnancy, either a full or empty bladder may be required at the time the amniocentesis is performed. Be sure to follow any instructions provided.
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. Amniotic fluid is contained within the amniotic sac and 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 it as urine. The amount of amniotic fluid increases over time and is constantly being absorbed and renewed. During this circulation process, cells from various parts of the fetus's body slough off into the fluid, and chemicals produced by the fetus are present as well. This is why samples of the fluid can be tested to evaluate fetal health. Several laboratory tests may be performed on the fluid to help identify various conditions and diseases (see "How is the test used?" below).
Recently, advances in testing technology and improved prenatal screening options, specifically non-invasive prenatal screening (NIPS), has led to a decrease in the use of diagnostic tests such as amniocentesis. NIPS screens a blood sample from a pregnant woman for fragments of cell-free DNA (cfDNA) produced by the placenta. It typically screens for certain chromosomal abnormalities, including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), 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 of a positive prenatal screening test or to test for conditions not covered by the screening test.
Amniotic fluid analysis may be used for several different purposes and the specific testing that is done depends on the reason for testing. The following list includes some of the more common ways that this analysis may be used for prenatal diagnosis:
To detect chromosomal abnormalities or genetic disorders
The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women should be given the option of having amniocentesis performed to detect chromosome abnormalities. A healthcare practitioner can help a pregnant woman weigh the pros and cons.
Due to the nature of this type of testing, chromosome analysis can also definitively determine the sex of a fetus.
To detect open neural tube defects (NTDs), such as spina bifida or anencephaly, or open abdominal wall defects. For example:
To detect Rh and other blood type incompatibilities
When a mother has been previously exposed through prior pregnancies or blood transfusions to "foreign" red blood cells (RBCs), she may develop antibodies to the proteins present on the outer layer of the foreign red cells (antigens). If the antigens are present on the red blood cells of the fetus (inherited from the father), then the mother's RBC antibodies can cross the placenta and bind to and destroy the fetus's red blood cells, causing hemolytic anemia. An affected fetus can develop hemolytic disease of the newborn. One of the most commonly encountered incompatibilities are due to ABO and Rh group differences, but there are many other blood types that may also cause this problem.
Typically, if a fetus has been tested and is known to have Rh or blood type incompatibility with the mother, the fetus is monitored non-invasively using imaging scans. However, if the fetus shows signs that a blood transfusion is needed, then tests for bilirubin (also known as delta OD450) may be performed to help evaluate the severity of the hemolytic anemia. This is an invasive test and is not performed routinely. Bilirubin is an orange-yellow pigment, a waste product primarily produced by the normal breakdown of heme. Heme is a component of hemoglobin, which is found in red blood cells (RBCs).
To detect fetal infections
There are a few tests that can be performed on amniotic fluid to detect infections that are passed from mother to baby during pregnancy (congenital infections). Some of these infections may have serious consequences for the developing fetus. A few examples include tests for:
To evaluate fetal lung maturity
Recent guidelines from ACOG no longer recommends routine use of fetal lung maturity in clinical decision-making. However, testing to evaluate fetal lung maturity may be performed in select situations, for example, if a woman is very unsure of her dates and is at an increased risk for premature delivery or there is a sudden medical need for a preterm delivery. Various substances are measured in amniotic fluid (lamellar body count, phosphatidylglycerol (PG), lecithin/sphingomyelin (L/S) ratio). The goal of the testing is to determine the risk of a life-threatening condition called neonatal respiratory distress syndrome (RDS).
Genetic amniotic fluid analysis may be offered as part of second trimester prenatal testing and is performed primarily between 15 and 20 weeks of pregnancy. In particular, testing is recommended when:
Testing may be ordered to monitor bilirubin levels when there is a known Rh or blood type incompatibility between the fetus and mother and monitoring of the fetus indicates a blood transfusion may be needed.
An amniotic fluid analysis may be performed in late pregnancy to diagnose a fetal infection.
Fetal lung maturity amniotic fluid testing may be ordered woman is unsure of her dates and is at an increased risk for premature delivery or there is a sudden medical need for a preterm delivery.
Genetic tests, chromosome analysis and testing for neural tube defects
Because these results can be complex, women should discuss their test results not only with their healthcare practitioner but also ideally with someone who has expertise in genetics, such as a genetic counselor or maternal-fetal medicine specialist.
If a chromosomal abnormality or a genetic disorder is detected by diagnostic testing, then the baby likely will have the associated condition. However, many chromosomal disorders have a range of severity, and simply knowing a diagnosis does not help predict the condition's severity or prognosis. It should also be noted that not every genetic disorder or chromosomal abnormality will be detected with this testing.
If an increased alpha fetoprotein suggests an abnormality, such as an open neural tube defect or abdominal wall defect, then additional testing and imaging may be performed to determine the severity of the condition and the best course of action.
Rh or other blood type incompatibility
Increasing bilirubin levels in the amniotic fluid in the case of fetal-maternal blood type incompatibility indicate increasing destruction of red blood cells and the likelihood that the fetus will have hemolytic disease of the newborn, requiring treatment before or after birth, depending on the severity.
Results of cultures of the amniotic fluid as well as tests for various viruses will indicate whether an infection is present.
Fetal lung maturity
Although rarely used these days, if testing indicates that the a fetus's lungs have not yet matured, then a healthcare practitioner may take measures to attempt delaying delivery, use medications to promote lung maturity, or when necessary to begin treating the baby immediately after birth.
Amniocentesis is the removal of a small amount of fluid (about an ounce) from the sac that surrounds a developing fetus using a needle and syringe. Before the procedure, ultrasound is used to find the position of the fetus in the womb and is continuously used throughout the procedure to ensure that the needle remains safely away from the baby. Prior to inserting the needle, the skin on the mother's abdomen is cleaned and sometimes a local anesthetic is applied to the surface of the skin or injected into the skin.
During the procedure, the needle is carefully inserted through the walls of the abdomen and the uterus so that it just enters the thin-walled sac of amniotic fluid that surrounds the developing fetus. A small amount of amniotic fluid is discarded to ensure that no maternal contamination is present within the needle, and then the fluid is drawn up into the syringe and sent to a laboratory for analysis. Depending on the specific tests being performed, results may be available within a few days to up to 4 weeks or, in the case of fetal lung maturity testing, within a few hours.
There is a slight risk with amniocentesis that the needle inserted into the amniotic sac might puncture the baby, cause a small amount of amniotic fluid leakage following the procedure, cause a uterine infection, or, in rare cases, cause a miscarriage. The risk for a miscarriage associated with this procedure is considered to be 1/300 to 1/500, which is much lower than a woman's risk (at any age) to have a baby with a chromosomal abnormality.
This is a highly personal decision, and one best made by a woman and her healthcare provider working together. The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women be given the option of having amniocentesis performed. Although the associated risk is low, it must be weighed against the desire for information that would be gained by testing. Speaking with a genetic counselor may also be helpful in learning more about the various types of testing available and what to expect from the information that can be learned through them.
No, the testing requires specialized equipment and training to interpret. It needs to be performed in a laboratory and may need to be sent to a reference laboratory. However, the procedure for collecting the amniotic fluid can be done in an outpatient setting, such as a healthcare practitioner's office.
It is in the sense that if there is too much or too little fluid present, it may indicate a problem with the fetus, the placenta, or both. Ultrasound is typically used to estimate the amount of amniotic fluid that is present.
Maternal or fetal blood contamination and stool from the fetus (meconium) in the amniotic fluid can affect some chemical test results.
An alternative to amniotic fluid analysis for chromosomal analysis and genetic testing is chorionic villus sampling (CVS), which can be performed earlier, between 10 and 12 weeks of pregnancy. This first trimester procedure collects a small placenta tissue sample at the site of implantation and carries about the same risks as amniocentesis when performed by an experienced practitioner. CVS cannot, however, detect neural tube defects or abdominal wall defects, and occasionally may provide a chromosomal result that is representative of the placenta, but not the fetus (confined placental mosaicism).
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