Amniocentesis (also referred to as amniotic fluid test or AFT) is a medical procedure used in prenatal diagnosis of chromosomal abnormalities and fetal infections, and also used for sex determination in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for genetic abnormalities. The most common reason to have an "amnio" is to determine whether a baby has certain genetic disorders or a chromosomal abnormality, such as Down syndrome. Amniocentesis (or another procedure, called chorionic villus sampling (CVS) can diagnose these problems in the womb. Amniocentesis is usually done when a woman is between 16 and 22 weeks pregnant. Though all women should be offered the option of having an amnio, women who choose to have this test are primarily those at increased risk for genetic and chromosomal problems, in part because the test is invasive and carries a small risk of misscarriage. Developed by Richard Dedrick, this process can be used for prenatal sex discernment and hence this procedure has legal restrictions in some countries.
Before the start of the procedure, a local anesthetic can be given to the mother in order to relieve the pain felt during the insertion of the needle used to withdraw the fluid. After the local anesthetic is in effect, a needle is usually inserted through the mother's abdominal wall, then through the wall of the uterus, and finally into the amniotic sac. With the aid of ultrasound-guidance, a physician punctures the sac in an area away from the fetus and extracts approximately 20ml of amniotic fluid.
If used for prenatal genetic diagnosis, fetal cells are separated from the extracted sample. The cells are grown in a culture medium, then fixed and stained. Under a microscope the chromosomes are examined for abnormalities. The most common abnormalities detected are Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Turner syndrome (monosomy X). In regard to the fetus, the puncture heals and the amniotic sac replenishes the liquid over the next 24–48 hours.
Indications and results
Early in pregnancy, amniocentesis used for diagnosis of chromosomal and other fetal problems such as:
- Down syndrome (trisomy 21)
- Trisomy 13
- Trisomy 18
- Fragile X
- Rare, inherited metabolic disorders
- Neural tube defects (anencephaly and spina bifida) by alpha-fetoprotein levels.
Amniocentesis can predict fetal lung maturity, which is inversely correlated to the risk of infant respiratory distress syndrome. In pregnancies of greater than 30 weeks, the fetal lung maturity may be tested by sampling the amount of surfactant in the amniotic fluid. Several tests are available that correlate with the production of surfactant. These include the lecithin-sphingomyelin ratio ("L/S ratio"), the presence of phosphatidylglycerol (PG), and more recently, the surfactant/albumin (S/A) ratio. For the L/S ratio, if the result is less than 2:1, the fetal lungs may be surfactant deficient. The presence of PG usually indicates fetal lung maturity. For the S/A ratio, the result is given as mg of surfactant per gm of protein. An S/A ratio <35 indicates immature lungs, between 35-55 is indeterminate, and >55 indicates mature surfactant production(correlates with an L/S ratio of 2.2 or greater).
Amniocentesis can also be used to detect problems such as:
- Infection, in which amniocentesis can detect a decreased glucose level, a Gram stain showing bacteria or an abnormal differential count of white blood cells.
- Rh incompatibility
- Decompression of polyhydramnios
An emerging indication for amniocentesis is in the management of preterm rupture of membranes where measurement of certain amniotic fluid inflammatory markers may be helpful. If amniotic fluid IL-6, a marker of inflammation, is elevated, the fetus is at high risk and delivery should be considered.
Risks and drawbacks
Amniocentesis is performed between the 15th and 20th week of pregnancy; performing this test earlier may result in fetal injury. The term "early amniocentesis" is sometimes used to describe use of the process between weeks 11 and 13.
Complications of amniocentesis include preterm labor and delivery, respiratory distress, postural deformities, chorioamnionitis, fetal trauma and alloimmunisation of the mother (rhesus disease). Studies from the 1970s originally estimated the risk of amniocentesis-related miscarriage at around 1 in 200 (0.5%). Three more recent studies from 2000-2006 estimated the procedure-related pregnancy loss at 0.6-0.86%. A more recent study (2006) has indicated this may actually be much lower, perhaps as low as 1 in 1,600 (0.06%). Unlike the previous studies, the number in this study only reflects the loss that resulted from amniocentesis complications and excluded the cases when parents decided for an abortion following the test results. In contrast to amniocentesis, the risk of miscarriage from chorionic villus sampling (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.
Amniocentesis and stem cells
A potential benefit of using amniotic stem cells over those obtained from embryos is that they side-step ethical concerns among pro-life activists by obtaining pluripotent lines of undifferentiated cells without harm to a fetus or destruction of an embryo. These stem cells would also, if used to treat the same individual they came from, sidestep the donor/recipient issue which has so far stymied all attempts to use donor-derived stem cells in therapies.
Artificial heart valves, working tracheas, as well as muscle, fat, bone, heart, neural and liver cells have all been engineered through use of amniotic stem cells. Tissues obtained from amniotic cell lines show promise for patients suffering from congenital diseases/malformations of the heart, liver, lungs, kidneys, and cerebral tissue.
- Chorionic villus sampling
- Percutaneous umbilical cord blood sampling
- Prenatal diagnosis
- Amniotic stem cells
- Amniotic fluid
- The word amniocentesis itself indicates precisely the procedure in question, Gr. ἀμνίον amníon being the "inner membrane round the foetus" and κέντησις kéntēsis meaning "pricking", i.e. its puncture in order to retrieve some amniotic fluid.
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