Definition and Overview
An amniocentesis, also known as an amniotic fluid test or simply AFT, is a procedure used to test the amniotic fluid inside a pregnant woman’s womb. It is used for diagnostic and screening purposes, and is strictly used when serious fetal problems are suspected. The procedure detects the presence of chromosomal abnormalities and birth defects and diagnoses health problems, but carries certain risks. Thus, doctors carefully weigh its potential benefits against possible risks and complications before it is even recommended to the patient.
Who should undergo and expected results
An amniocentesis is a special diagnostic test performed only if:
- There is an increased risk of fetal abnormalities or birth defects are suspected
- The placental membranes ruptured prematurely
One indicator of the presence of such problems is when a woman receives abnormal results on the triple screen test, a routine blood test that measures a pregnant woman’s AFP, hCG, and Estriol levels. These are the different substances uniquely produced by the body during pregnancy.
- AFP, or alpha-fetoprotein, is a protein produced by the fetus
- hCG, or human chorionic gonadotropin, is the hormone produced by the placenta
- Estriol, an estrogen hormone produced collectively by the fetus and the placenta
If the results of the triple screen test are abnormal, the patient’s health care provider may recommend an amniocentesis to test a pregnant woman’s amniotic fluid, which surrounds the developing fetus inside the womb. The goals of the procedure are:
- Detect chromosomal abnormalities, such as Down, Edwards, and Turner syndromes
- Detect neural tube defects, such as spina bifida or anencephaly
- Diagnose genetic disorders, such as cystic fibrosis
- Diagnose inherited metabolic disorders
- Determine the gender of the fetus
- Detect fetal or uterine infections
- Diagnose Rh disease
- Assess the severity of fetal anemia
- Determine paternity through DNA testing (the amniotic fluid contains the fetus’ DNA)
- Assess the maturity of the fetus’ lungs – This is one of the few reasons that may require an amniocentesis during the third trimester or shortly before the expected delivery date.
The test has up to 99% accuracy rate in detecting chromosomal abnormalities, neural tube defects, and genetic conditions and can determine paternity with the same accuracy rate. Its only limitation is that it cannot tell whether the birth defects are mild or severe, which creates the need for other tests, including advanced ultrasound scans.
In many instances, the amniocentesis helps determine whether the fetus requires lifesaving treatment. For example, if neural tube defects are found, the parents can consider pursuing fetal surgery. If genetic conditions are detected, they may also begin planning for and anticipate the special needs of their child when born.
How the procedure works
An amniocentesis, which is often performed in an obstetrician’s clinic or the hospital, is usually scheduled during the second trimester of pregnancy, or more specifically when the fetus is between 15 and 20 weeks old. In some less common cases, it can also be performed as early as the 11th to 13th weeks; however, this is best avoided as performing the test too early raises the risk of fetal injury. There is usually no need to use or repeat it during the third trimester of pregnancy.
The test involves collecting some of the fluid from within the amniotic sac after an ultrasound scan that allows the doctor to determine where it is the safest to inject the needle. The doctor then inserts a needle through the patient’s abdomen and into her fetus to draw around 20ml of amniotic fluid sample.
Although the collection of fluid may take just five minutes, the whole test may last for up to 45 minutes including the ultrasound scan and other preparations.
The obtained fluid sample is sent to the laboratory to be analyzed. The results are released within a few days to a couple of weeks and are discussed by the obstetrician to the patient during a follow-up appointment.
Possible risks and complications
An amniocentesis is an invasive procedure that carries certain risks to both the mother and the baby. Thus, it is used only when deemed absolutely necessary.
Some of the risks associated with the procedure include:
- Uterine infection
- Breaking of the water
- Premature labor
- Respiratory distress
- Postural deformities
- Fetal trauma
- Rhesus disease, or alloimmunization of the mother
The test itself may cause some pain when the needle is inserted through the skin, and another sharp pain when it penetrates the uterus. If necessary or if the patient requests for it, she may be given a local anesthetic before the procedure. A sonogram is also used to make sure that the needle does not come into contact with the fetus.
After the procedure, the mother may also experience some fluid leakage, abdominal cramps, and minor discomfort around the site where the needle was inserted.
The fetus suffers no late effects, as the amniotic sac automatically seals the puncture site and replenishes the fluid within 24 to 48 hours.
Despite these risks, amniocentesis is deemed safer than chorionic villus sampling or CVS, another test that detects chromosomal and genetic disorders.
It is extremely important for the pregnant woman to discuss and carefully consider the risks and benefits of the procedure with her obstetrician before going through with it. In some countries, the test is subject to some legal restrictions.
Simpson JL, Holzgreve W, Driscoll DA. Genetic counseling and genetic screening. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2012:chap 10.
Simpson JL, Richards DA, Otao L, Driscoll DA. Prenatal genetic diagnosis. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2012:chap 11.