Amniocentesis

Amniotic fluid analysis may 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:

To detect chromosomal abnormalities or genetic disorders

• Chromosome analysis, a cytogenetics test that may also be called karyotyping. Chromosome analysis will detect chromosomal abnormalities associated with a variety of disorders.It evaluates the 22 paired chromosomes and the sex chromosomes (X, Y) in the nucleus of cells cultured from those collected in 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), 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
  • 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. 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 could 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

To detect birth defects
Testing for open neural tube defects (NTDs), such as spina bifida or anencephaly, detects chemicals that may be present in abnormal quantities when a fetus has an anatomic abnormality. For example:

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

To evaluate 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:

• Lamellar body count
• Phosphatidylglycerol (PG)
• Lecithin/sphingomyelin (L/S) ratio

To detect 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 blood 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 ABO group differences, 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 a fetus.

To evaluate fetal distress
Evaluation of amniotic fluid color may be indicative of fetal distress.

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:

• Cytomegalovirus (CMV)
• Toxoplasmosis
• Parvovirus B19 (Fifth disease)
• Cultures for bacterial infections

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 analysis may be 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 Down syndrome screen or second trimester maternal serum screen, such as an increased or decreased alpha-feto protein (AFP) level
• A woman had a previous child or pregnancy with a chromosomal 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.

An amniotic fluid analysis may be performed in late pregnancy to check for fetal distress and to diagnose a fetal infection.

Genetic tests, chromosome analysis and testing for birth defects
Women should discuss their test results with their health practitioner and with a genetic counselor.

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 genetic conditions cannot be ruled out.Not every genetic disorder or chromosomal abnormality will be detected with this 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 testing indicates 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 are deemed high enough, then the baby may be safely delivered without increased risk of complications from lung immaturity.

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, requiring treatment depending on the severity.

Fetal distress or infection
Evaluation of amniotic fluid color:

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

Cultures of the amniotic fluid will indicate whether or not an infection is present.

Both blood contamination and stool from the baby (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 placenta tissue sample at the site of implantation and carries about the same risks as amniocentesis. CVS cannot, however, detect neural tube defects.

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