Amniotic Fluid Testing

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.

• Chromosome analysis—performed as either a cytogenetics test that can also be called karyotyping or as a chromosomal microarray analysis. Both methods detect chromosome abnormalities associated with a variety of chromosome disorders. The testing evaluates the 22 paired chromosomes and the sex chromosomes (X, Y) in cells from the sample of amniotic fluid and can be used to diagnose a variety of disorders, including:
  • Down syndrome (Trisomy 21), caused by an extra chromosome 21 in all or most cells of the body
  • Edwards syndrome (Trisomy 18), caused by an extra chromosome 18
  • Patau syndrome (Trisomy 13), caused by an extra chromosome 13
  • Klinefelter syndrome, the most common sex chromosome abnormality in males; caused by an extra X chromosome
  • Turner syndrome, caused by missing one X chromosome in females

    Due to the nature of this type of testing, chromosome analysis can also definitively determine the sex of a fetus.

• Genetic testing, also called molecular testing, evaluates fetal DNA to look for specific gene mutations that are associated with specific genetic disorders. Molecular testing may be offered because of a family history of a specific inherited disorder or because of specific fetal ultrasound abnormalities that are known to have a genetic cause.

To detect open neural tube defects (NTDs), such as spina bifida or anencephaly, or open abdominal wall defects. For example:

• AFP (alpha-fetoprotein) is elevated in amniotic fluid with neural tube defects.
• Acetylcholinesterase is increased with neural tube defects and other anatomic abnormalities.

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:

• TORCH: toxoplasmosis, rubella, cytomegalovirus (CMV), herpes simplex virus (HSV)
• Parvovirus B19
• Cultures for bacterial infections

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:

• A woman has an abnormality on a first trimester Down syndrome screen, non-invasive prenatal screening (NIPS) using cell-free DNA, or second trimester maternal serum screen
• A woman had a previous child or pregnancy with a chromosomal abnormality or genetic disorder
• A parent has an inherited disorder or both parents have a known gene for an inherited disorder
• An abnormality has been detected on a fetal ultrasound

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.

Fetal infections
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.

• Genetic and chromosomal abnormalities cannot be prevented, only diagnosed.
• The risk for open neural tube birth defects can be minimized by a woman getting plenty of folic acid prior to and during pregnancy.
• The number of women with Rh sensitization has greatly decreased since injections of Rh immune globulin to prevent the formation of Rh antibodies became routine in prenatal and postnatal care.