Prenatal Diagnostic Testing: Amniocentesis Procedure

Amniocentesis is a prenatal test in which a small amount of amniotic fluid is removed from the sac surrounding the fetus for testing. The sample of amniotic fluid (less than one ounce) is removed through a fine needle inserted into the uterus through the abdomen, under ultrasound guidance. The fluid is then sent to a laboratory for analysis. Different tests can be performed on a sample of amniotic fluid, depending on the genetic risk and indication for the test.

Before the start of the procedure, a local anesthetic can be given to the mother in order to relieve the pain felt during the insertion of the needle used to withdraw the fluid. After the local anesthetic is in effect, a needle is usually inserted through the mother's abdominal wall, then through the wall of the uterus, and finally into the amniotic sac.

With the aid of ultrasound-guidance, a physician punctures the sac in an area away from the fetus and extracts approximately 20ml of amniotic fluid. If used for prenatal genetic diagnosis, fetal cells are separated from the extracted sample. The cells are grown in a culture medium, then fixed and stained. Under a microscope the chromosomes are examined for abnormalities.

The most common abnormalities detected are Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Turner syndrome (monosomy X). In regard to the fetus, the puncture heals and the amniotic sac replenishes the liquid over the next 24–48 hours.

If the results of your amniocentesis are normal, your baby most likely does not have genetic or chromosomal abnormalities. In the case of maturity amniocentesis, normal test results will assure you that your baby is ready to be born.

Abnormal results may mean that your baby has a genetic problem or serious birth defect. Discuss all test results with your doctor and your partner so that you can make an informed decision about whether or not to continue your pregnancy.

Prenatal Diagnostic Testing: What pregnant women can expect when coming for an amniocentesis at BC Women's Hospital.

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