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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. It is usually offered when there may be an increased risk for genetic defects in the pregnancy.
Amniocentesis can be done as soon as there is enough amniotic fluid surrounding the fetus that a sample can be removed safely. Early amniocentesis can be performed as early as 13 weeks gestation. Standard amniocentesis is usually performed between 15 and 20 weeks gestation. Results take about two weeks. Often, genetic counseling is done before amniocentesis, or other types of genetic testing are offered.
The procedure is straightforward, 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pathogens into the amniotic sac from the needle and the puncture wound not healing properly after the syringe has been withdrawn which leads to leakage or infections. Serious complications can result in miscarriage. Other possible complications include preterm labour and delivery, respiratory distress, postural deformities, fetal trauma and alloimmunisation (rhesus disease).
Amniocentesis is not 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. The test is typically not performed later than the second trimester.
Amniocentesis is considered to be an elective procedure, but it is usually recommended for women older than 35. Age 35 is customarily when physicians start recommending this test. This is to balance the risk of miscarriage and the higher risks of having a child with chromosomal abnormalities. 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. [How to reference and link to summary or text] In cases of family histories with chromosomal abnormalities or problematic pregnancies, some doctors will suggest younger women to take the test even though they do not fit the typical age profile.
The risk of amniocentesis-related miscarriage is generally thought to be 1 out of every 200, but a recent study has shown the risk to be 1 out of every 1,600.
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 fetal 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 syringe is usually inserted in to the mother's abdominal wall or at the end of the vagina, and through the uterus wall. 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. The puncture wound should close up by itself, just like any shots one would get, and the amniotic sac should then automatically replenish the liquid over a day or so.
Once the amniotic fluid is extracted, fetal cells are separated from the fluid using a centrifuge, and the lab will check the chromosomes in the cells for abnormalities. Various types of genetic testing could be done but it would be up to the individual lab doing the work to decide on the list of diseases they are able to detect and look for. They could either be cytogenetics screening or simply looking for abnormal genes. The doctor might order additional screening to look for particular genetic signature if there is family history of that particular disease. In any case, it is impossible to perform a comprehensive screening of all known human genetic diseases at this time, regardless of which lab you and your doctor choose.
(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 fetal cells, some labs would use the remainder amniotic fluid to find indications of non-genetic diseases the fetus might have. Levels of chemical substances, (such as alpha-fetoprotein) can be measured to detect other health problems such as hydrocephalus.
- Chorionic villus sampling
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