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Amniocentesis: learning more about the fetus

With all the advances in medicine, doctors and their patients can learn a lot about the development of a fetus, and one of the tools that's commonly used is amniocentesis.

The amniotic sac is the fluid-filled structure inside the pregnant uterus within which the baby lives. Fetal cells, proteins, and fetal urine freely move within this sac. Amniocentesis, is a procedure which involves removing a small portion of the amniotic fluid which surrounds the fetus by a fine needle. So it's not the fluid that's examined, but the cells in the fluid.

Doctors then examine the fluid for genetic defects, infections, or for fetal maturity, if indicated.

The criteria for amniocentesis would be age, prior history, an abnormal fetus being picked up at the time of an ultrasound, or an abnormal triple screen..

What the procedure involves:

The test involves localization of a pocket of fluid. The patient's abdomen is prepped to maintain sterility, and then a long, thin-bore needle is passed into the abdomen under ultrasound guidance, with or without local anesthesia. Some doctors use local, most of them in fact do not, and while visualizing the needle with the ultrasound, going into the fluid, they actually then aspirate or remove approximately 30 cc of amniotic fluid. That is two tablespoons. That is an insignificant amount, because at that stage of pregnancy, there is about 150-300 cc of fluid. So the baby really does not miss this, and it is regenerated, and there is not a problem just from removing that very small amount of fluid. The ultrasound helps guide the needle to the proper place.

The fluid is put into tubes, usually about three different tubes. When it comes out, it's a clear to yellow color. Look at it, you can't see the fetal cells present in the fluid. The tubes are taken to a laboratory, and put in what's called a centrifuge, which is a very fast spinner.
The cells, being heavier, go to the bottom. A little white button is formed on the bottom. These cells are taken out and put in tiny little culture dishes to help them grow and divide. Because it's only dividing cells where chromosomes can be seen. These cells are put on slides, stained, and seen under the microscope.

A level of the alpha-fetoprotein can be achieved, which is a protein produced by the fetal liver, and will be elevated if there's an opening in the fetus. An elevated AFP blood level may indicate the possibility that the fetus has spina bifida, a defect in brain and spinal cord development. A low AFP level may indicate the potential for Down's syndrome. When abnormal levels of AFP are found, further evaluation with ultrasound and amniocentesis can be done to look for birth defects and chromosomal abnormalities.

With The fluid we can measure the alpha-fetoprotein level right away and get an answer regarding neural tube defect. But the chromosomes and any type of genetic testing that involves DNA, requires growing the cells in the laboratory. It takes about a week to 10 days to get the results of the chromosomes. DNA testing, takes a little bit longer, maybe a few days longer, maybe even a couple of weeks longer.

Amniocentesis can be used to:

  • Diagnose a large number of genetic and chromosomal abnormalities in the fetus. Testing for Tay-Sachs disease, cystic fibrosis, sickle cell anemia, and other inherited disorders can be performed on samples of amniotic fluid.

  • In addition, it is helpful in the diagnosis of the severity of Rh incompatibility.

    Chromosome analysis: Samples from genetic tests are analysed to look for a chromosomal abnormality. This test result shows one extra chromosome number 21, indicating Down's Syndrome in the fetus.

  • lung maturity- Amniocentesis testing for lung maturity can help the doctor make decisions regarding timing of delivery for women in premature labor, or those with diabetes or other medical conditions requiring delivery.

  • neural tube defects (such as, spina bifida).

  • DNA testing is available for many diseases.

Ask your obstetrician or geneticist if you have a question about a specific disease.

Risk involved:

There is a slight chance of infection or injury to the fetus. There is even a smaller chance of miscarriage. This test is typically performed when a problem is suspected, so the benefits outweigh the risk.

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