|Other names||Amniotic fluid test (AFT)|
Amniocentesis (also referred to as an amniotic fluid test or, informally, an "amnio") is a medical procedure[note 1] used primarily in prenatal diagnosis of chromosomal abnormalities and fetal infections as well as for sex determination. In this procedure, a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amniotic sac surrounding a developing fetus. The fetal DNA is then examined for genetic abnormalities.
The most common reason to have an amniocentesis performed is to determine whether a fetus 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. These prenatal examinations can prove helpful to expectant guardians, as they allow for evaluating the fetal health status and the feasibility of treatment.
An amniocentesis is performed when a woman is between 15 and 20 weeks gestation. 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 miscarriage. This process can be used for prenatal sex discernment and hence this procedure has legal restrictions in some countries.
Several researchers worked on the development of amniocentesis for fetal sex determination in the 1950s.
Between 1959 and 1967,a scientist developed the new technique of amniocentesis for clinical assessment of fetal wellbeing in utero. He presented his results at the William Blair-Bell Memorial Lecture at the RCOG in London in 1965 and was awarded an MD from the University of Manchester for this work. He also described amniocentesis techniques, as well as other details about amniotic fluid, in the chapter "The Liquor Amnii" in the 1970 and 1977 editions of Scientific Foundations of Obstetrics and Gynaecology.
Up to the mid-1970s amniocentesis procedures were done 'blind‘. Doctors Jens Bang and Allen Northeved from Denmark were the first to report amniocentesis done with the guide of an ultrasound in 1972. Chorionic villus sampling (CVS) was first performed by Italian biologist Giuseppe Simoni in 1983. Real-time ultrasound is now used during all invasive procedures because it provides for the safety of the fetus and accuracy of results.
Early in pregnancy, amniocentesis is used for diagnosis of chromosomal, genetic, and other fetal problems such as:
- Down syndrome, also known as Trisomy 21
- Trisomy 13
- Trisomy 18
- Sex chromosome aneuploidies, also known as sex chromosome anomalies
- Fragile X
- Neural tube defects (anencephaly and spina bifida) by alpha-fetoprotein levels.
- Rare, inherited metabolic disorders
Amniocentesis can be used to test fetal lung development. Problems with lung development can increase the risk of infant respiratory distress syndrome. Fetal lung development can be tested by sampling the amount of surfactant in the amniotic fluid in pregnancies greater than 30 weeks. Several tests are available, including the lecithin-sphingomyelin ratio ("L/S ratio"), the presence of phosphatidylglycerol (PG), or 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, 35-55 is indeterminate, and >55 indicates mature surfactant production.
Amniocentesis can be used to diagnose Rh incompatibility, a condition when the mother has Rh-negative blood and the fetus has Rh-positive blood. Early detection is important to treat the mother with Rh immune globulin and to treat her baby for hemolytic anemia.
Decompression of polyhydramnios
Polyhydramnios, or the accumulation of amniotic fluids which leads to increase risk of cesarean section, can be relieved via decompression amniocentesis. Amniocentesis can also be used to diagnose potential causes of polyhydramnios.
Preterm rupture of membranes
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.
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.
Amniotic fluid embolism has also been described as a possible outcome. Additional risks include amniotic fluid leakage and bleeding. These two are of particular importance because they can lead to spontaneous abortion in pregnant patients.
The prenatal diagnosis of chromosomal abnormalities can have social drawbacks as technology changes the way people think about disability and kinship. In one sense, amniocentesis offers a window of control and in another, an anxiety-provoking responsibility to make rational decisions about complex, emotional and culturally contingent issues.
This procedure is typically performed in the outpatient setting by a team of providers. With ultrasound guidance, a needle is inserted into the abdomen at an angle through the muscle, into the uterus and into the amniotic cavity. There are various methods to obtain samples including a single needle and double needle technique. These techniques have their own variations in how they are performed including guidance of needle insertion location, and angle of needle insertion. The collected amniotic fluid is then submitted for laboratory testing for chromosomal abnormalities and the puncture site heals with time.
Before the procedure is performed, the fetus is analyzed via ultrasound to determine if it is viable and for any abnormalities. The ultrasound determines the location of the placenta, fetal position and movements, and characteristics of the amniotic fluid. This information is utilized to determine the type of needle used and how the procedure should be performed.
The abdomen is treated and cleaned with antiseptic before the procedure. Sterile gel is also used on the abdomen before scanning with a sterile ultrasound probe. These measures are taken to reduce infection risk. The tools used in the procedure are coated with heparin to prevent clotting.
With the aid of ultrasound guidance, a needle is inserted through the mother's abdominal wall, then through the wall of the uterus, and finally into the amniotic sac. The physician then punctures the sac in an area away from the fetus and extracts approximately 20ml of amniotic fluid. This procedure can be performed with a single needle or a double needle technique based on individualized patient factors and physician preference. From the 20 ml of amniotic fluid, the first 2 ml is typically discarded due to mixture with maternal blood cells to ensure high quality fluid sampling.
Post procedure recommendations and analysis
If used for prenatal genetic diagnosis, fetal cells are separated by centrifugation 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). The sample is also analyzed for fetal infection and for intra-amniotic inflammation through infection studies.
After the procedure, the patient is instructed to take house rest for the initial 24 hours post-procedure however normal activities are allowed such as personal hygiene. In regard to the fetus, the puncture seals and the amniotic sac replenishes the liquid over the next 24–48 hours. One week after the procedure, the mother will have a follow up appointment for ultrasound analysis to confirm fetal viability and to assess healing of the puncture site.
A potential benefit of using amniotic stem cells over those obtained from embryos is that they side-step ethical concerns among anti-abortion 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.
Prenatal sex discernment and sex-selective abortion
The Child Sex Ratio (Number of Females per Thousand Males) has been more prominent for males in countries such as India and China. Sex-selective abortion has been a leading cause for this higher ratio of male to female births. Techniques such as Ultrasound, Amniocentesis, Chorionic Villus Sampling and Preimplantation genetic diagnosis can provide information as to the sex of the fetus, and in some countries their use for prenatal sex discernment is highly restricted or banned. According to the Pre-Conception and Pre-Natal Diagnostic Techniques Act (1994) of India, Amniocentesis is not banned rather it is regulated to find information other than the sex of the fetus.
- Chorionic villus sampling
- Amniotic fluid
- Amniotic stem cells
- Elective genetic and genomic testing
- Percutaneous umbilical cord blood sampling
- Prenatal diagnosis
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