Also Known As
Fetal Lung Maturity Tests
Formal Name
Amniotic Fluid Analysis
This article was last reviewed on
This article waslast modified on 25 April 2023.
At a Glance
Why Get Tested?

To detect and diagnose some birth defects, genetic diseases, and chromosome abnormalities in a fetus, especially if antenatal screening tests are abnormal; to help diagnose and monitor haemolytic disease in a fetus

When To Get Tested?

Between 15 and 20 weeks of gestation to test for genetic diseases, chromosome abnormalities, and open neural tube defects; when it is suspected that a fetus has haemolytic disease, about every 14 days

Sample Required?

A sample of amniotic fluid is obtained using a procedure called amniocentesis

Test Preparation Needed?

You may be instructed to have a full or empty bladder prior to amniocentesis

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Amniotic fluid surrounds, protects, and nourishes a growing fetus during pregnancy. It allows the baby to move relatively freely, keeps the umbilical cord from being compressed, and helps maintain a stable temperature. Contained within the amniotic sac, amniotic fluid 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 2 pints or 1 litre at 36 weeks of pregnancy. The fluid is absorbed and continually renewed.

The fetus swallows and inhales amniotic fluid and releases urine into it. Cells from various parts of the fetus's body and chemicals produced by the fetus are present in the amniotic fluid. This allows the fluid to be sampled and tested to evaluate fetal health.

Amniotic fluid analysis encompasses a range of tests that can be performed to evaluate the health of a fetus. The tests are performed on a sample of amniotic fluid that is obtained through a procedure called amniocentesis. These tests may be performed on samples obtained between 15 and 20 weeks of pregnancy to test for some genetic diseases, chromosome abnormalities such as Down syndrome, and neural tube defects. It may also be done when it is suspected that a fetus has a blood type incompatibility with the mother and is at risk for developing haemolytic disease.

For genetic testing and chromosome analysis, fetal cells in the amniotic fluid are cultured and grown for several days in the laboratory, then are analysed. Some genetic tests can be performed directly on the amniotic fluid.

Accordion Title
Common Questions
  • How is it used?

    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:

    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
  • When is it requested?

    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:

    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.


  • What does the test result mean?

    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.

  • Is there anything else I should know?

    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.

  • What is amniocentesis and how is it performed?

    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.

  • Can amniotic fluid be tested to look for infections?

    Yes, 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, and parvovirus B19 (Fifth disease) and cultures for bacterial infections.

  • Can disorders detected through amniotic testing be prevented?

    Genetic and chromosomal abnormalities cannot be as they are inherited conditions. The risk for open neural tube birth defects can be minimised by a woman getting plenty of folic acid prior to and during pregnancy. The number of women with Rh sensitisation has greatly decreased since injections of Rh immune globulin (RhoGam) to prevent the formation of Rh antibodies became routine.

  • Should everyone have amniotic fluid analysis?

    The decision is best made between a woman, her partner, and her doctor. The slight risk associated with amniocentesis must be weighed against the information gained. It is not considered standard of care for low-risk patients in contrast to high-risk patients.

  • Can amniotic fluid testing be performed in my doctor's office?

    No, it requires specialised equipment and training to interpret. It needs to be performed in a laboratory and may need to be sent to a reference laboratory.

  • Is there any other way to test the fetus's health?

    Sometimes a sample of blood is taken from the umbilical cord in late gestation pregnancies (percutaneous umbilical blood sampling, PUBS, or cordocentesis). Fetal cells also can be present in the mother's bloodstream. They are sometimes obtained by separating them from a sample of the mother's blood. Rarely, a tissue sample may be obtained from the fetus.

  • Is the amount of amniotic fluid present important?

    Yes. Separate testing may be performed to evaluate the quantity of amniotic fluid that surrounds a fetus. Too little (oligohydramnios) early in a pregnancy can cause birth defects or miscarriage. Late in the pregnancy, it can cause poor fetal growth or even cause a stillbirth. Too much amniotic fluid (polyhydramnios) may cause few symptoms. When it is severe, it may affect the pregnant woman's breathing and increase the risk of premature birth and postpartum haemorrhage.