amniocentesis - Risks, Procedure

A diagnostic procedure performed at 14–18 weeks gestation which involves withdrawal of a small sample of amniotic fluid from within the uterus using a needle inserted through the abdominal wall under ultrasound guidance. Amniotic fluid is in close contact with the fetus, and contains cells that can be analysed to reveal many congenital fetal abnormalities. These include Down's syndrome and spina bifida. The procedure can also reveal the sex of the fetus and other normal features of development. It carries a small risk of inducing an abortion.

Amniocentesis, or an Amniotic Fluid Test (AFT), is a medical procedure used for prenatal diagnosis, in which a small amount of amniotic fluid is extracted from the amnion around a developing fetus.

Amniocentesis can be done as soon as there is enough amniotic fluid surrounding the fetus that a sample can be removed safely. Often, genetic counseling is done before amniocentesis, or other types of genetic testing are offered.

Risks

The procedure itself is straight forward, but some complications with the procedure include the introduction of pathogens into the amniotic sac from the needle and the puncture wound not healing properly after the syringe has been withdrawn which leads to leakage or infections.

Amniocentesis is half as risky to the child as chorionic villus sampling although villus sampling can be done earlier.

Amniocentesis is not performed earlier because there is not as much amniotic fluid when the fetus is younger. There is a greater risk to the fetus if fluid is taken out.

Amniocentesis is considered to be an elective procedure, but it is usually recommended for women older than 35. The risk of miscarrying a healthy fetus resulting from amniocentesis is greater than the risk that the fetus has a serious developmental disease if the mother is younger than 35 years old.

Procedure

The basic idea of the procedure is to somehow get a hold of the fetus' cells to examine the Chromosomes and/or DNA from these cells in order to rule out genetic abnormalities that could lead to birth defects.

The most accessible fetus cells are in the amniotic fluid, where one would find traces of the fetus' skin and other cells that have sloughed off the fetus during its growth. Amniotic fluid is often obtained using a long syringe, guided by ultrasound. The physician would aim for an area of the amniotic sac that is away from the fetus so to avoid stabbing it. A small amount of amniotic fluid then gets sucked out and the syringe is withdrawn.

Once the amniotic fluid is extracted, fetus cells are separated from the fluid using a centrifuge, and the lab will check the the chromosomes in the cells for abnormalities.

(A side effect of cytogenetic analysis is determination of the sex of the fetus with 100% accuracy, although the sex of the fetus can be discovered using the much more common and less intrusive means of examining ultrasound images.)

In addition to the fetus cells, some labs would use the remainder amniotic fluid to find indications of non-genetic diseases the fetus might have.