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. It is usually offered when there may be an increased risk for genetic conditions (i.e. Down syndrome, sickle-cell disease, cystic fibrosis, etc) 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, is offered.
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 in 400 and 1 in 200.
Amniotic fluid is often obtained using a long syringe, guided by ultrasound. The syringe is usually inserted through the mother's abdominal wall, and also at the end of the vagina, and amniotic fluid sucked out. Also, in women younger than 35, the risk of miscarriage through the procedure is greater than the risk of carrying a child with chromosomal abnormalities. As well as genetic testing, levels of other substances, (such as alpha-fetoprotein) can be measured to detect other health problems. Amniocentesis cannot detect all possible genetic problems or birth defects.
In the amniotic fluid, there are traces of the baby's skin and other cells that have sloughed off the baby during its growth. Chromosomes and/or 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.)
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