by Farrah Tahar

Created on: June 24, 2008

Amniocentesis is a medical procedure used in prenatal diagnosis to detect genetic abnormalities and fetal infections during pregnancy. This procedure is also commonly known as the amniotic fluid test.

During the procedure, a needle is inserted through the mother's abdominal wall or through the vagina, then through the wall of the uterus and into the amniotic sac. The physician conducting the procedure makes use of an ultrasound to direct the needle to an area away from the fetus, and extracts a small amount of fluid from the amniotic sac. This fluid contains fetal cells, proteins, and fetal urine, which can be sent for analysis to determine if there are any genetic abnormalities. Amniocentesis does not detect birth defects such as heart malformations or a cleft lip or palate.

The three most common abnormalities tested for are Downs' Syndrome, Trisomy 18 and spina bifida, although a large number of other tests for other genetic abnormalities are possible with the fetal chromosomes obtained through amniocentesis. In addition to these, the amniotic fluid may also be examined for indications of non-genetic diseases that the fetus may have developed, through analysis of chemical composition of the fluid.

Amniocentesis can be performed as soon as sufficient amniotic fluid surrounds the fetus to allow a sample to be recovered relatively safely, usually no earlier than the 14th week of pregnancy. The amount of amniotic fluid removed during the procedure is usually replenished within a day and the puncture heals naturally.

There are, however, risks involved with amniocentesis. While the procedure is fairly routine, possible complications include infection of the amniotic sac from the needle, and failure of the puncture to heal properly. Serious complications may result in miscarriage, preterm labor and delivery, respiratory distress, postural deformities, fetal trauma and alloimmunisation (rhesus disease).

The risk of amniocentesis-related miscarriage is generally thought to be 1 in 200, although a recent study has indicated this may actually be much lower, perhaps 1 in 1,600. Because of the risk involved, only expectant mothers at risk of having babies with genetic abnormalities are recommended to have the procedure. These include mothers over the age of 35, who already have children with Down Syndrome or other chromosomal disorders.

Amniocentesis is very accurate in detecting chromosomal abnormalities and gender of the fetus. However, the procedure does occasionally fail. The fetal cells harvested by amniocentesis may not grow in culture and no chromosome data can be obtained. Additionally, the doctor may unable to extract fluid from the uterus during amniocentesis due to a variety of technical reason. In these cases, the procedure may be repeated.

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