Amniocentesis is a medical procedure carried out on pregnant women. It is an invasive prenatal diagnostic test in which a small amount of amniotic fluid is removed from the uterus and analyzed for detecting various fetal disorders.
What is Amniotic Fluid?
Amniotic fluid is a clear yellow liquid that surrounds the developing fetus in the mother’s womb. It is also called liquor amnii.
It is formed within the amniotic sac from the mother’s plasma (fluid part of blood cells) as it diffuses across the fetal membranes owing to forces of osmosis and hydrostatic pressure. By the beginning of the second trimester, when the fetal kidneys begin to function, fetal urine also contributes to the formation of this fluid.
Its main function is to serve as a cushion for the growing fetus against various types of injuries. It also serves to facilitate the exchange of nutrients, water, and biochemical products between mother and fetus.
In addition to important nutrients, hormones, and antibodies; amniotic fluid also contains cells shed by the fetus. These cells contain genetic information about the fetus which can be harvested and analyzed for chromosomal disorders. Amniotic fluid also contains alpha-fetoprotein, a substance produced by the fetus; levels of which can indicate the presence of spinal and other disorders in the fetus.
When is Amniocentesis Carried Out?
It is usually carried out between 15-20 weeks of gestation for detecting various fetal disorders including chromosomal defects. When carried out at this stage, parents can also be offered the option of terminating the pregnancy in case of a positive result.
In some cases, the procedure is carried out in the third trimester for detecting fetal infections, assessing well being and /or maturity of the fetus.
Difference Between Chorionic Villus Sampling (CVS) and Amniocentesis
Similar to amniocentesis, CVS is also a prenatal invasive diagnostic technique for diagnosing various fetal disorders. In this test, a sample of chorionic villi (placental tissue) is obtained and analyzed.
It is the preferred technique before 15 weeks of gestation.
As compared to amniocentesis, CVS carries a slightly increased risk of fetal loss. However, it can’t detect neural tube defects or Rh incompatibility.
An amniocentesis may be used for genetic and chromosome testing in the second trimester of pregnancy. The presence of one or more of the following conditions is an indication for amniocentesis –
- Family history or previous child with a genetic disease or chromosomal or metabolic disorder
- Risk of open neural tube defects such as spina bifida
- Abnormal maternal screening tests, such as maternal serum alpha-fetoprotein level (MSAFP), triple test, quadruple test or abnormalities detected in level I and level II ultrasound.
- Advanced maternal age > 35 years
Read more about Genetic Inheritance: Modes and Significance
Amniocentesis is used to detect the following disorders:
• Down syndrome (trisomy 21)
• Patau syndrome (trisomy 13)
• Edwards syndrome (trisomy 18)
• Fragile X syndrome
• Turner syndrome (monosomy X)
• Sickle cell anemia
• Hemoglobinopathies including thalassemia major
Neural tube defects
Anencephaly and spina bifida by alpha-fetoprotein levels of amniotic fluid.
Rare inherited metabolic disorders
Testing for Fetal lung maturity
It is usually carried out in the third trimester by detecting the level of surfactant in the amniotic fluid. If premature birth is anticipated, it can give information about whether the baby’s lungs are mature enough for birth. Several parameters can be tested that correlate with the production of surfactant. These include
Phosphatidylglycerol (PG) levels
Lecithin-sphingomyelin ratio (“L/S ratio”)
Ratio < 2: 1 indicates reduced surfactant levels
Surfactant/albumin (S/A) ratio
Ratio < 35 indicates reduced surfactant and immature lungs. Ratio > 55 indicates mature lungs while the ratio between 35-55 indicates an indeterminate result.
Diagnosis of fetal infection
Amniocentesis may also be used to detect fetal infections. The fluid obtained can be subjected to various tests including total cell count, differential cell counts, glucose levels, gram stain, etc.
Diagnosis of Rh sensitization
Amniocentesis can be used to evaluate the severity of anemia in babies who have Rh sensitization ( a condition in which an Rh-negative mother’s immune system produces antibodies against Rh-positive blood cells of the fetus.
Read more about Rh Immune Globulin
Read more about Fetal Maternal Haemorrhage
In the case of polyhydramnios, (excessive accumulation of amniotic fluid during pregnancy), amniocentesis may be used as a form of treatment to drain excess amniotic fluid from the uterus.
Stem cell harvesting
Stem cells obtained from amniotic fluid can be stored and used to cure a number of diseases of the child in the future, if the need arises.
It can also detect the sex of the fetus. Since sex determination tests are banned in some countries, the test also carries legal implications.
In case of preterm rupture of membranes, amniotic fluid may be used to measure levels of certain inflammatory markers such as IL-6. Increased levels of inflammatory markers indicate a risk to the fetus and premature delivery should be considered in such patients.
Amniocentesis is usually performed on an outpatient basis or as part of the patient’s stay in a hospital. The procedure is as follows
- Informed consent is obtained from the patient.
- The patient is asked to lie down on an examination table and keep her hands behind her head.
- An ultrasound is performed to check the fetal heart rate, the position of the placenta, fetus, and umbilical cord, and to locate a pocket of amniotic fluid away from the fetus.
- After checking the vital signs (blood pressure, heart rate, and breathing rate), the abdominal skin is cleansed thoroughly with an antiseptic.
- A local anesthetic is injected into the abdominal skin.
- Under ultrasound guidance, a long, thin hollow needle is inserted through the mother’s abdominal wall, into the wall of the uterus, and finally into the amniotic sac
- A small amount of amniotic fluid is then withdrawn into a syringe. The amount depends upon the type and number of tests to be performed. Usually, about 20 ml of amniotic fluid is removed.
- The withdrawn fluid is transferred in a clean sterile and light-protected container.
- The needle is removed.
- An adhesive bandage is placed over the needle site.
- The amniotic fluid is sent to the lab for examination.
- Both the mother and the fetus need to be monitored after the procedure.
- The mother’s vital signs
- The fetal heart rate
- Checked periodically for an hour or longer.
- In case the mother is Rh-negative, she may be given Rh immunoglobulin to prevent Rh-negative mother’s antibodies from reacting with Rh-positive fetal cells.
- Some amount of cramping may be felt during or after the procedure. Pain-killers may be administered for relieving this discomfort.
- In case the mother feels dizziness or nausea, she may be instructed to rest on her left side.
- The patient is instructed to rest at home. Strenuous activity, exercise, lifting heavy objects or engaging in sexual activity should be avoided for 1 or 2 days after the procedure.
The site of puncture seals and heals in due course and the amniotic sac replenishes the fluid over the next 1-2 days.
The patient is asked to report back immediately in case the patient experiences any of the following symptoms
- Any bleeding or leaking of amniotic fluid from the needle puncture site or the vagina
- Fever and/or chills
- Abdominal pain which is more severe than cramping
- Changes in the activity level of the fetus ie, reduced fetal movements (beyond 20-24 weeks of pregnancy)
Lab Testing of Amniotic Fluid
The amniotic fluid sample is sent to a special genetics lab for analysis of prenatal genetic disorders. Fetal cells are separated from the amniotic fluid sample. The cells are grown in a suitable culture media, then fixed and stained. The chromosomes of these cells are examined under a microscope for various chromosomal abnormalities.
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Alpha-fetoprotein present in the fluid can also be measured to diagnose neural tube defects, such as spina bifida.
Levels of other substances can be measured to diagnose a variety of metabolic or genetic conditions.
How long does it take for the results?
Tests for detecting chromosomal disorders take about 2-3 weeks. The detection of various biochemical substances is done within a few hours.
Procedural Risks and Complications
- Cramping and/ or severe abdominal pain
- Bleeding or leaking of amniotic fluid from the puncture site or the vagina
- Infection (chorioamnionitis)
- Alloimmunisation of the mother
- The procedure may cause some of the fetus’s blood cells to enter and mix with the maternal bloodstream.
- In Rh-negative mother and Rh-positive fetus, maternal antibodies against Rh-positive blood cells are produced.
- To prevent this, Rh immunoglobulin is given to Rh-negative mothers after the procedure.
- Fetal trauma
- Postural deformities in the fetus
- Miscarriage: The risk of miscarriage is less than one percent after an amniocentesis in the second trimester of pregnancy.
- Preterm labor and delivery
- Amniotic fluid embolism resulting in maternal respiratory distress
Depending on test results, counseling with a genetics specialist may be recommended. In case of confirmation of a particular fetal disorder, the parents and family may be given the option of medical termination of pregnancy. Psychological support to the mother must be offered in all such cases.