Amniotic Fluid Analysis

Amniotic fluid analysis may used for several different reasons 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:

Chromosome abnormalities or genetic disorders

• Chromosome analysis, a cytogenetics test that may also be called karyotyping. Chromosome analysis will detect chromosome abnormalities associated with a variety of disorders. It evaluates the 22 paired chromosomes and the sex chromosomes (XY) in the nucleus of cells cultured from those collected in the sample of amniotic fluid and can be used to diagnose a variety of chromosomal disorders. Some of these include:
  • Down syndrome (Trisomy 21), caused by an extra chromosome 21 in all or most cells of the body
  • Edwards syndrome (Trisomy 18), associated with severe mental retardation; 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
  • As a byproduct, chromosome analysis can also definitely determine the sex of the fetus.
• Genetic testing, also called molecular testing. It looks at fetal DNA to identify specific gene mutations and diagnose a variety of inherited diseases. Each test is ordered separately based upon the parents' personal and family histories. If a specific gene mutation is known to be present in the mother's or father's family, then that specific gene mutation can be tested for. While there are hundreds of genetic tests that can be performed, only a few are associated with relatively common disorders. Typically, only the most common mutations are tested; therefore, a fetus can still have an inherited disorder even though genetic testing did not identify one. A few examples of genetic tests more frequently performed include those for:
  • Cystic Fibrosis
  • Tay-Sachs disease
  • Canavan disease
  • Familial dysautonomia
  • Sickle cell anemia
  • Thalassemia

Birth defects
Testing for open neural tube defects (NTDs), such as spina bifida (spinal defect) or anencephaly (brain does not develop), detects chemicals that may be present in abnormal quantities when the fetus has an anatomic abnormality.

• AFP (alpha-fetoprotein)—increased with neural tube defects
•  Acetylcholinesterase—increased with neural tube defects and also other anatomic abnormalities

Fetal Lung Maturity
Testing to evaluate fetal lung maturity may be performed if a woman is at an increased risk for premature delivery or an early birth is necessary for the baby or mother's health. The tests are based upon the presence of adequate protective liquid substances called surfactants in the lungs, which are necessary for proper lung function. If there are insufficient surfactants, a newborn may develop life-threatening neonatal respiratory distress syndrome (RDS). Tests include:

• Fluorescence polarization (FPOL, surfactant/albumin ratio)
• Lamellar body count
• Phosphatidylglycerol (PG)
• Lecithin/sphingomyelin (L/S) ratio (less common than FPOL)

Rh and other blood type incompatibilities and diseases
When a mother has been previously exposed through prior pregnancies or blood transfusions to red blood cell antigens that she does not have on her own red cells, she may develop antibodies against those red cell antigens (become sensitized). If the antigens are present on the red blood cells of the fetus (inherited from the father), then the fetus is at risk for a maternal-fetal incompatibility. The mother's antibodies can cross the placenta and bind to and destroy the fetus's red blood cells, causing hemolytic anemia. An affected fetus can be born with hemolytic disease of the newborn. One of the most commonly encountered incompatibilities is due to Rh, but there are many other blood types that may also cause this problem.

• Tests for bilirubin may be performed on a regular basis, starting at about 25 weeks of pregnancy, to detect, evaluate and monitor the severity of the hemolytic anemia in the fetus.

Fetal Distress
Evaluation of amniotic fluid color – may be indicative of fetal distress

• Green tinged – indicates that meconium, the fetus's first stool, has been released
• Yellow to amber – may indicate bilirubin in the fluid
• Red tinged – blood from the mother or the fetus

While amniocentesis is safe and has been performed for many years, it is an invasive procedure that poses a slight risk of injury to the fetus and of miscarriage. For this reason, it is not performed routinely with each pregnancy.

Genetic amniotic fluid testing is offered as part of second trimester prenatal testing and is performed primarily between 15 and 20 weeks gestation if:

• A woman is 35 years of age or older
• A woman has an abnormality on a first trimester prenatal screen or second trimester (triple/quad) screening test, such as an increased or decreased alpha-feto protein level
• A previous child had a chromosome abnormality or birth defect
• There is a strong family history of a specific genetic disorder
• A parent has an inherited disorder or both parents have a gene for an inherited disorder
• An abnormality has been detected on a fetal ultrasound

Fetal lung maturity amniotic fluid testing is ordered when there is a risk of premature delivery, at any time after 32 weeks gestation.

Biochemical testing is sometimes ordered to monitor bilirubin levels when a woman has been sensitized or it is suspected that she has become sensitized (has developed antibodies) to red blood cell antigens and there may be an Rh or other blood type incompatibility with the fetus. In this case, serial testing for bilirubin may be performed, usually about every 14 days.

Genetic tests, chromosome analysis and testing for birth defects
Women should discuss their test results with their doctor and frequently with a genetic counselor as well. If a chromosomal abnormality or a genetic disorder is detected, then the baby likely will have the associated condition. However, test results may not predict the condition's severity or prognosis. Normal results make it less likely that a fetus has an inherited condition, but all conditions cannot be ruled out. Not every genetic disorder or chromosome abnormality will be detected with testing.

If an increased or decreased alpha fetoprotein suggests a structural abnormality, such as an open neural tube defect, then additional testing and imaging may be performed to determine the severity of the condition and the best course of action.

Fetal lung maturity
If tests indicate that there are low levels of surfactants, then a fetus's lungs have not yet matured and measures can be taken to attempt to delay delivery, to promote lung maturity, and – when necessary – to treat the baby as soon as it is born. If the levels of surfactants is deemed high enough, then the baby may be safely delivered without the increased risk of complications.

Rh or other blood type incompatibility
Increasing bilirubin concentrations in a fetus with a fetal-maternal blood type incompatibility indicate increasing destruction of red blood cells (RBCs) and the likelihood that the fetus will be born with hemolytic disease of the newborn.

Both blood contamination and stool from the baby (meconium) in the amniotic fluid can affect some chemical test results.

Performed on peripheral blood obtained from the mother, the first trimester screen for Down syndrome and the second trimester triple marker or quad marker screen for Down syndrome and open neural tube defects assess the risk of the fetus having these conditions but are not diagnostic. In most cases, the subsequent amniotic fluid tests will be normal. Only a small percentage of those with an abnormal blood screening test result will actually have an affected baby.

For chromosomal analysis and genetic testing earlier than 15 weeks in a pregnancy, chorionic villus sampling (CVS) can be performed instead of amniocentesis. This first trimester procedure collects a placenta tissue sample at the site of implantation between 10 and 12 weeks gestation and carries about the same risk as amniocentesis. CVS cannot, however, detect neural tube defects.