Amniocentesis is a prenatal test in which a small sample of amniotic fluid is drawn from the uterus. This sample contains genetic material that can be tested for chromosomal abnormalities and hereditary disorders.

Amniotic fluid surrounds the developing fetus inside the amniotic sac. Fetal secretions are mixed in the amniotic fluid, this fluid is also sometimes swallowed by the fetus.


When is amniocentesis recommended?

Amniocentesis is not commonly offered to all women, it is recommended only in cases of a risk for fetal defects, abnormal screening test results, or a maternal age of over 35 (that is associated with an increased risk of Down syndrome).

Since amniocentesis poses a small risk, healthcare providers discuss the procedure with the parents and help them understand its advantages and possible complications.


How is the test performed?

The procedure is performed by an obstetrician (and in some cases by a Gynecologic ultrasonography specialist). The test usually takes around thirty minutes.

To withdraw the sample, a thin needle is inserted into the uterus through the abdomen, under ultrasound guidance. 

Before the procedure, an ultrasound transducer is placed on the abdomen to identify the position of the fetus, amniotic fluid, and placenta. This is done to avoid injuries during the procedure. The test carries a small risk for loss of the pregnancy. Miscarriage is reported in around 0.5% of cases. However, advancements in medical technology and skilled professionals have helped reduce this risk.

More possible complications include premature labor (with all the risks that this entails), Chorioamnionitis (amniotic fluid infection), and needle injuries to the fetus.

Amniocentesis is considered safer than chorionic villus sampling (CVS). CVS carries a risk of miscarriage of around 1%.


What diseases can amniocentesis detect?

The amniotic fluid sample collected in amniocentesis contains fetal cells. Since amniotic fluid contains fetal urine secretions, it can also contain epithelial cells- which contain genetic material.

These cells are separated from the amniotic fluid and genetically tested. One of the procedures performed is a karyotype test, that can detect chromosomal defects such as Down syndrome, Turner syndrome, trisomy 18, and trisomy 13. 

When the fetus is at risk for a specific disorder, a FISH test or PCR test can also be performed to detect smaller scale genetic defects. Amniocentesis can also be used as part of paternity testing – to prove a relation between the father and fetus.

In rare cases of CMV infection, amniocentesis can help find out if the fetus has become infected with the virus. Infection can have a critical influence on the decision to continue the pregnancy – infection in early stages of pregnancy has been linked to fetal brain and nervous system defects, although this is not certain. In cases of a risk for CMV infections, amniocentesis is usually performed around week 22 into the pregnancy to detect the virus in the amniotic fluid (and not in fetal cells).

Amniocentesis can also be used in the assessment of fetal lung maturity. This procedure detects substances that indicate lung maturity. Test results are important in advanced stages of pregnancy. In cases of a need for preterm birth, amniocentesis can help assess if the fetus is at risk of lung diseases.


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