Amniocentesis
Amniocentesis or amniotic fluid test is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is necessary to conclusively diagnose the majority of genetic disorders, with amniocentesis being the gold-standard procedure after 15 weeks' gestation.
In this procedure, a thin needle is inserted into the abdomen of the pregnant woman. The needle punctures the amnion, which is the membrane that surrounds the developing fetus. The fluid within the amnion is called amniotic fluid, and because this fluid surrounds the developing fetus, it contains fetal cells. The amniotic fluid is sampled and analyzed via methods such as karyotyping and DNA analysis technology for genetic abnormalities.
An amniocentesis is typically performed in the second trimester between the 15th and 20th week of 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 0.1% to 0.3% risk of pregnancy loss with the risk of pregnancy loss being much higher if the surgery is performed before 15 weeks. However, the American College of Obstetricians and Gynecologists recommends that all women be offered prenatal assessment for aneuploidy, or the presence of an abnormal number of chromosomes, by either genetic screening or diagnostic testing independent of maternal age or risk factors. There are relative contraindications to performing an amniocentesis, however no absolute contraindications have been identified.
Physicians have used the process of inserting a needle transabdominal into the uterus to extract amniotic fluid for the management of hydramnios, or excess amniotic fluid, as early as the late 1800s.
Medical uses
Amniocentesis may be performed for both diagnostic and therapeutic reasons.Diagnostic indications
Genetic diagnosis
The American College of Obstetricians and Gynecologists recommends that all women be offered prenatal assessment for aneuploidy by either genetic screening or diagnostic testing independent of maternal age or risk factors. Prenatal genetic screening is intended to identify patients who are at increased risk of having a fetus with a genetic condition while prenatal genetic diagnostic testing detects whether a specific genetic condition is present in the fetus with high accuracy. Amniocentesis, along with chorionic villus sampling, are examples of prenatal diagnostic tests. Amniocentesis or chorionic villus sampling is necessary to conclusively diagnose the majority of genetic disorders, with amniocentesis being the gold-standard procedure after 15 weeks' gestation. Transabdominal chorionic villus sampling is an alternative to amniocentesis if genetic diagnostic testing is to be performed in the first trimester between ten and 15 weeks' gestation. It is important to note that prenatal genetic testing cannot identify all possible fetal genetic abnormalities or their outcomes. When deciding on whether to perform invasive genetic diagnostic testing such as amniocentesis, patients and their physicians should participate in a shared-decision-making process that takes into account a patient's individual risk profile and preferences.Amniocentesis may be offered to certain patients at higher risk of having a fetus with a genetic disorder. Factors that may place patients at increased risk of fetal genetic disorders include older maternal or paternal age, parental carrier of a balanced chromosomal rearrangement, parental aneuploidy or aneuploidy mosaicism, parental carrier of a genetic disorder, prior child with a structural birth defect, previous fetus or child with autosomal trisomy or sex chromosome aneuploidy, ultrasonographic evidence of a fetus with structural abnormalities, and a high-risk genetic screening test result.
Analysis of samples obtained from amniocentesis is accomplished via karyotyping and DNA analysis technology. Examples of conditions that may be detected include chromosomal abnormalities such as common aneuploidies like trisomy 13, trisomy 18, trisomy 21, Turner syndrome, and Klinefelter syndrome. Micro-deletions or micro-duplications, such as DiGeorge syndrome and Cri-du-Chat syndrome, sex-linked disorders, such as hemophilia and Duchenne muscular dystrophy, and specific genetic mutations previously demonstrated to be present in a family or suspected based on fetal ultrasound findings, such as sickle cell disease, Tay–Sachs disease, neurofibromatosis, and cystic fibrosis.
Assessment of fetal lung maturity
Amniocentesis has traditionally been performed to assess the extent of fetal lung development in the context of medical and obstetrical complications, with the intention of delivering the fetus if fetal lung maturity is demonstrated. Lack of fetal lung maturity increases the risk of infant respiratory distress syndrome.Fetal lung development can be tested by sampling the amount of surfactant in the amniotic fluid obtained via amniocentesis. Several tests are available, including the lecithin-sphingomyelin ratio, the presence of phosphatidylglycerol, and the surfactant/albumin ratio. An L/S ratio of 2.0 is associated with a lower incidence of infant respiratory distress syndrome.
The efficacy of performing amniocentesis for the assessment of fetal lung maturity has been called into question. The Eunice Kennedy Shriver National Institute of Child Health and Human Development and the Society for Maternal-Fetal Medicine's joint statement on the matter states that "if significant maternal or fetal risks exist, delivery should occur regardless of biochemical maturity and if delivery could be deferred owing to absence of pulmonary maturity there is not a stringent indication for prompt delivery." Based on this rationale, the risks associated with amniocentesis, and the limited indications, performing amniocenteses for assessing fetal lung maturity may become obsolete except in instances where gestational age is unknown.
Assessment and management of infection
, or intraamniotic infection, is an infection of any combination of the amniotic fluid, placenta, fetus, fetal membranes, or decidua. The gold standard for diagnosing chorioamnionitis is via a gram stain, glucose level, or culture of the amniotic fluid obtained via amniocentesis. However, in clinical practice, performing an amniocentesis for the purpose of diagnosing chorioamnionitis is rare and instead diagnosed based on clinical signs to ensure prompt treatment and avoid invasive prenatal testing.Amniocentesis can be used to detect other congenital infections such as cytomegalovirus, hepatitis B, parvovirus B19, and toxoplasmosis.
Assessment of severity of Rh isoimmunization
The Rh factor is an inherited protein found on the surface of red blood cells. If the mother is Rh negative and the father is Rh positive, a fetus has at least a 50% chance of being Rh positive. Rh incompatibility occurs when a mother has Rh-negative blood and her baby has Rh-positive blood. If the red blood cells of an Rh positive fetus cross into their Rh negative mother's blood flow, the mother is at risk of Rh sensitization, a process in which maternal antibodies form against red blood cell Rh antigens. Events causing fetomaternal hemorrhage, such as miscarriage, trauma to the abdomen during pregnancy, ectopic pregnancy, induced abortion, invasive pregnancy procedures, labor, and birth can lead to fetal Rh positive red blood cells entering the Rh negative maternal circulation. Rh sensitization typically does not cause problems during the first pregnancy of an Rh negative woman. However, in a subsequent pregnancy with an Rh positive fetus, the formed antibodies can cross the placenta and attack the red blood cells of the fetus, a process called Rh isoimmunization that can result in a condition known as Rhesus disease or hemolytic disease of the fetus and newborn.Hemolytic disease of the fetus and newborn can lead to varying degrees of fetal anemia, which can potentially have devastating consequences including newborn jaundice, kernicterus, hydrops fetalis, and intrauterine fetal demise. RhD immune globulin is administered to prevent maternal Rh sensitization in Rh negative patients that are non-sensitized to Rh antigens.
In the past, serial amniocenteses has been used to monitor isoimmunized pregnancies by measuring the levels of bilirubin, a byproduct of red blood cell breakdown, present in amniotic fluid using spectrophotometry. However, ultrasound detecting middle cerebral artery peak systolic velocity has now replaced serial amniocenteses for the management of isoimmunized pregnancies.
Amniocentesis is an invasive prenatal test that has the potential to cause maternal and fetal blood cell mixing, which can worsen Rh isoimmunization.
Therapeutic indications
Decompression of polyhydramnios
is an excessive accumulation of amniotic fluid that can lead to serious perinatal and maternal outcomes such as intrauterine fetal demise, preterm labor, premature rupture of membranes, and cesarean delivery. Physicians have used the process of inserting a needle transabdominally into the uterus to extract excess amniotic fluid, also known as a reductive amniocentesis or decompression, for the management of polyhydramnios as early as the late 1800s.Reductive amniocentesis is currently indicated for severe cases of polyhydramnios only.