Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Amniocentesis (also referred to as amniotic fluid test or AFT), is a medical procedure used in prenatal diagnosis of genetic risk factors, in which a small amount of amniotic fluid, which contains fetal tissues, is extracted from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for genetic abnormalities. Amniocentesis is not performed for every pregnancy, but is generally done when an increased risk of genetic defects in the fetus is indicated, by mother's age (over 35 years is common), family history of genetic defects, or other factors.
A syringe is usually inserted through the mother's abdominal wall or at the end of the vagina, and through the wall of the uterus into the amniotic sac. With assistance from ultrasound a physician aims for an area of the sac that is away from the fetus and extracts a small amount of amniotic fluid for testing. The puncture heals, and the amniotic sac automatically replenishes the liquid over a day or so. After the amniotic fluid is extracted, the fetal cells are separated from it using a centrifuge, and cell chromosomes are examined for abnormalities. Various genetic testing may be performed, but the three most common abnormalities tested for are Down's syndrome, Trisomy 18 and spina bifida. 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. Often, genetic counseling is offered in conjunction with amniocentesis.
In addition to the fetal cells, some tests may examine the amniotic fluid for 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.
An alternative test to examine the genome of the developing fetus is chorionic villus sampling, or CVS.
A side benefit of genetic analysis through amniocentesis or CVS is that the sex of the fetus may be determined with 100% accuracy.
Although the procedure is routine, possible complications include infection of the amniotic sac from the needle, and failure of the puncture to heal properly, which can result in leakage or infection. Serious complications can result in miscarriage. Other possible complications include 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. In contrast, the risk of miscarriage for CVS is believed to be approximately 1 in 100, although CVS may be done up to four weeks earlier, and may be preferable if the possibility of genetic defects is thought to be higher. 
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