Amniocentesis, or the amniotic fluid test, is an invasive prenatal diagnostic test that detects chromosomal abnormalities in the fetus.
Those with a higher risk of giving birth to children with a serious health condition or abnormality include:
- women aged over 35 years
- women with a medical history of an inherited condition such as sickle cell anemia or muscular dystrophy
- women who have undergone a previous pregnancy with genetic problems such as Down’s syndrome
The amniocentesis test is 98 to 99 percent accurate for testing chromosomal abnormalities, neural tube defects, and genetic disorders. Around 200,000 tests are carried out in the United States (U.S.) each year.
Amniocentesis is often carried out between 15 and 18 weeks of gestation.
The amniotic sac, or amnion, surrounds the fetus during pregnancy. The sac contains a fluid called amniotic fluid. The fetus is completely submerged in this fluid.
In the test, an ultrasound guides a 22-gauge spinal needle to a safe place in the amniotic sac.
The needle extracts between 10 and 20 milliliters (ml) of amniotic fluid from the sac, and the fluid is sent for testing. This represents about 1 ml of fluid per week of gestation.
The whole procedure takes around 45 minutes, but the extraction of the fluid itself takes less than 5 minutes.
How is the fluid analyzed?
In the lab, the fetal DNA is examined for genetic abnormalities.
The fluid carries fetal tissue, including skin cells and fetal waste products, so it can be used to assess whether the fetus is developing normally, or if it is at risk of developing a serious health condition or abnormality.
Each cell from the fetus contains a complete set of DNA, or genetic information.
Analyzing these cells enables a doctor to assess fetal health. They will be able to detect any potential problems.
If the doctor diagnoses a problem at this stage, it may be possible to treat it before the fetus leaves the womb.
Amniocentesis may detect Rh disease, which can lead to severe anemia in the fetus. If so, a blood transfusion may be possible.
If there is a significant problem, the parents may decide to terminate the pregnancy.
Amniocentesis can detect several conditions.
- Down syndrome: An extra chromosome, known as trisomy-21, can affect a person’s physical features, mental development, and ability to learn.
- Sickle cell anemia: The red blood cells have an unusual shape or texture. Red blood cells carry oxygen around the body.
- Thalassemia: The body’s ability to create red blood cells is poor.
- Trisomy 13 (Patau’s syndrome): The extra chromosome, trisomy 13, causes a serious condition in which the newborn survives just for a few days. It is very rare.
- Trisomy 18, or Edward’s syndrome: Trisomy-18 is an extra chromosome that causes severe physical and mental abnormalities.
- Fragile X, or Martin-Bell syndrome: This is a genetic syndrome that causes physical, cognitive, emotional, and behavioral challenges, which can range from mild to severe.
- Spina bifida and other neural tube defects.
In the later stages of pregnancy, amniocentesis can detect infection, Rh incompatibility, lung maturity prediction, and decompression of polyhydramnios.
Chorionic villus sampling (CVS) is an alternative to amniocentesis. Tissue is taken from the vascular fingers, or villi, of the chorion, a part of the placenta.
Amniocentesis is not a routine test. It is only carried out if there is a significant risk that the fetus will have a serious condition or abnormality.
The fetus is considered to be at risk of genetic or developmental issues when:
- There is a family history of, or a previous child with, a genetic condition, such as Down syndrome.
- There is a risk of open neural tube defects, as in spina bifida.
- The woman will be aged 35 years or older on the estimated date of delivery.
- The earlier triple test, a blood test taken between weeks 15 and 20, shows abnormal results. The triple test looks for high and low levels of alpha-fetoprotein, human chorionic gonadotropin (hCG), and estriol.
- There is a risk of a sex-linked genetic disease, such as hemophilia or Duchenne’s muscular dystrophy.
Amniocentesis can also assess lung maturity in cases where a premature birth may be a possibility.
The mother will normally be offered genetic counseling.
An amniocentesis can also determine paternity with 99 percent accuracy.
This diagnostic procedure is invasive, so it can carry some risks.
In the second trimester, there is about a 1 percent a risk of miscarriage or stillbirth. The risks are greater if the test is carried out before 15 weeks.
In the third trimester, it may lead to problems with fetal lung development.
Other risks include:
- bleeding or leaking of amniotic fluid
- preterm labor
Other factors can increase the risk.
- attempting amniocentesis at less than 14 weeks gestation
- the position of the fetus and the placenta
- the amount of amniotic fluid
- the mother’s anatomy
- the presence of twins or multiple fetuses
It is important to discuss with a doctor whether or not it is necessary to perform an amniocentesis because of the risks involved.
Amniocentesis is often covered by insurance as long as there is a medical indication for it, for example, if the expectant mother is over 35 years of age.
A written referral from a physician or a gynecologist may be necessary.