Amniocentesis 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. This is usually at about 14 weeks gestation, and is usually performed up to about 20 weeks gestation.
Amniocentesis is slightly less risky to the child than chorionic villus sampling although villus sampling can be done earlier. Amniocentesis done in the second trimester is often said to have a risk of fetal death between about 1/400 and 1/200.
Amniotic fluid is often obtained using a long syringe, guided by ultrasound. The syringe can be inserted through the mother's stomach, and also at the end of the vagina, and amniotic fluid sucked out.
In the amniotic fluid there are traces of the baby's skin and other cells that have sloughed off the baby while it's growing. DNA from these cells in the fluid are then analysed, usually to rule out genetic abnormalities which could cause birth defects. This can be cytogenetics or simply looking for abnormal genes. (A side effect of cytogenetic analysis is determining the sex of the fetus, although that is more commonly discovered through ultrasound.) As well as genetic testing, levels of other substances, (such as alpha-fetoprotein) can be measured to give an indication of the health of the child.
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