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
  • Edward 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 requested 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 anaemia
  • 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

These tests in amniotic fluid have largely been superseded by the use of detailed ultrasound examination usually carried out between 18 and 22 weeks.

Fetal Maternal 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 sensitised). 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 haemolytic anaemia. An affected fetus can be born with haemolytic 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 haemolytic anaemia in the fetus.

Fetal Distress
Evaluation of amniotic fluid colour – 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 has a high risk of chromosomal anomaly following routine antenatal screening (see first trimester prenatal screen, second trimester (quad) screening and Non-Invasive Prenatal Testing (NIPT) test. 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 scan

Biochemical testing is sometimes requested to monitor bilirubin levels when a woman has been sensitised or it is suspected that she has become sensitised (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 counsellor 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 alpha fetoprotein is seen during the Quad test which 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 Maternal Rh and other blood type incompatibilities
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 haemolytic disease of the newborn.

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

The first trimester screen or second trimester quad marker screen for Down syndrome, Edward Syndrome and Patau syndrome assess the risk of the fetus having these conditions but the tests are not diagnostic. Women with high risk results from first or second trimester screening are usually offered Non-Invasive Prenatal Testing (NIPT) before being offered chorionic villus sampling (CVS) or amniocentesis.

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 11and 13 weeks gestation and carries about the same risk as amniocentesis. CVS cannot, however, detect neural tube defects.

Amniocentesis is the removal of a small amount of fluid (about 10-15 milliliters or two to three teaspoons) from the sac that surrounds a developing fetus using a needle and syringe. Before the procedure, the doctor will use ultrasound to find the position of the fetus in the womb. The area on the mother's abdomen is cleaned and a local anaesthetic is applied to or injected into the skin.

During the procedure, the needle is inserted through the walls of the abdomen and uterus and into the thin-walled sac of fluid that surrounds the developing fetus. A small amount of amniotic fluid is withdrawn.

There is a slight risk with amniocentesis that the needle inserted into the amniotic sac may puncture the baby, cause a small amount of amniotic fluid leakage, cause an infection, and in rare cases cause a miscarriage.