Amniocentesis is the aspiration of amniotic fluid from the amniotic cavity and is usually used for prenatal diagnosis of aneuploidy or congenital diseases and infections. It is the most commonly performed invasive fetal test.
The fetus is surrounded by two layers, i.e., outer chorion, and inner amnion. Inside the amnion lies the amniotic cavity containing the amniotic fluid. During early gestation, the amnion can be seen ultrasonographically as a thin line. As the gestational age progresses, the amnion completely obliterates the chorionic cavity, which is usually by 12 to 14 weeks. Unfused membrane has been defined as a minimum 3 mm separation between the two membranes involving 50% of the amniotic cavity. Amniocentesis is hence done beyond 15 weeks. Early amniocentesis is associated with the tenting of the amniotic membrane causing higher chances of amniotic fluid leakage and fetal abnormalities. Amniocentesis that is done before membrane fusion is associated with a higher chance of failure to reach the amniotic fluid at first prick, thus increasing the number of pricks and complication rates.
There are no absolute contraindications for the procedure. Relative contraindications include:
The procedure is done under continuous ultrasound guidance. Apart from an ultrasonography machine, the following equipment is required:
Before Undergoing the Procedure
At the Time of the Procedure
After confirming the prerequisites and once the preparation is complete, the procedure is commenced. The technique can be performed by the freehand method under ultrasound guidance, or a guided needle attached to the probe can be used. A 20 gauge to 22 gauge spinal needle is used to enter the amniotic cavity under continuous ultrasound guidance. A firm entry into the amniotic cavity is recommended to prevent the tenting of the amniotic membrane. Once entry into the cavity is confirmed, amniotic fluid is slowly aspirated. The initial 1 ml to 2 ml of amniotic fluid is discarded because it has the highest chance of maternal cell contamination. Approximately 18 ml to 20 ml of amniotic fluid is required for karyotype testing, and 2 ml to 5 ml is required to test for enzyme deficiency testing. The needle is removed after adequate amniotic fluid has been obtained. Entry into the amniotic cavity through the placenta should generally be avoided because it increases the chances of bloody tap, especially in Rh-negative women. After the procedure, fetal cardiac activity is confirmed. Administration of anti-D is required in women with Rh-negative pregnancy.
In experienced hands, the risk of complications is minimal. Both maternal and fetal complications are experienced with amniocentesis.
The risk of complications is high when more than or equal to 3 pricks are used to obtain amniotic fluid. In ideal conditions, if an adequate fluid sample is not obtained in 2 pricks, the procedure should be abandoned for 24 hours, whereafter it can be re-attempted. In experienced hands, people performing more than 300 procedures/year; the risk is less. The risk of fetal loss is higher in women who are otherwise at a higher risk of miscarriage, such as women carrying fetuses with structural malformations, fibroids, retroplacental hematoma, obese women, women with vaginal infection at the time of the procedure. Amniocentesis in up to 86.0% of the patients was safe and free from any complications.
Amniocentesis is a confirmatory test to obtain fetal karyotype. It is easier to obtain karyotype after 15 weeks by amniocentesis. Term amniocentesis is associated with a high failure rate. In advanced gestational age, it is required to confirm fetal lung maturity; however, because of a fewer number of amniocytes culture and obtaining karyotype is difficult. Once the amniotic fluid is obtained, it is sent for a conventional cell culture report, which is obtained in 14 days. There are rapid chromosomal preparations available that give results in 1 to 2 days, including fluorescent in-situ hybridization (FISH) and quantitative fluorescence polymerase chain reaction (QF-PCR). The procedure is relatively safe, with fewer complications amongst experienced hands. The location of the placenta is an important factor for amniocentesis. While performing the procedure, we should try to avoid penetration of the placenta. The anterior and fundal placenta was associated with a higher number of complications, including multiple pricks, blood-stained liquor; however, it is not associated with an increase in the number of fetal loss rates. Passing the needle through the placenta is slightly associated with an increase in rates of preterm birth.
The decision to perform amniocentesis and convey the results to the couple requires communication between geneticists and fetal medicine experts. The patient has to be counseled by geneticists for them to know the true possibility of the fetus being affected by some genetic disease. Following the counseling session patient is referred for the procedure. While performing the procedure, coordination amongst the team is required. Once the needle is inside the amniotic cavity, the assistant should carefully withdraw the amniotic fluid so as the procedure is performed in minimal time possible and with minimal risk of needle displacement.
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