Amniocentesis

Article Author:
Aditi Jindal
Article Editor:
Chitra Chaudhary
Updated:
6/3/2020 2:53:50 PM
PubMed Link:
Amniocentesis

Introduction

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.

Anatomy and Physiology

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.[1] 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.[2]

Indications

  • Increased risk of fetal aneuploidy in the combined test, non-invasive prenatal test (NIPT), abnormal genetic sonography (positive major soft tissue marker, two positive minor soft tissue marker), previously affected fetus, family history of a balanced translocation.
  • Increased risk for genetic disease, i.e., an autosomal recessive disease with carrier status of both parents, or X-linked recessive diseases.
  • Maternal transmittable disease, i.e., TORCH infections, such as toxoplasmosis, rubella, cytomegalovirus, herpes simplex, and other organisms.[3]
  • Invasive tests on maternal request - usually not an indication but can be done in exceptional cases.
  • Advanced maternal age (older than 35 years) in itself is not an indication of invasive testing.[4]
  • To assess fetal lung maturity in late gestation.
  • Rh isoimmunized pregnancy to assess bilirubin in amniotic fluid and grade the severity of alloimmunization. It has largely been replaced by non-invasive tests, including the middle cerebral artery doppler.
  • Therapeutic: In hydramnios to relieve maternal discomfort and to instill intraamniotic drugs.[5]

Contraindications

There are no absolute contraindications for the procedure. Relative contraindications include:

  • Infections
  • Patients on anticoagulants. Oral anticoagulants should be stopped 48 to 72 hours before the procedure, and patients may be shifted to low molecular weight heparin. 

Equipment

The procedure is done under continuous ultrasound guidance. Apart from an ultrasonography machine, the following equipment is required:

  • Sterile swabs and drapes
  • Syringe 2 ml, 10 ml
  • Needle 20 gauge to 22 gauge
  • Containers for collection and sample transport
  • 5% Povidone-iodine solution

Preparation

Before Undergoing the Procedure

  • Couples should undergo genetic counseling.
  • Written consent should be obtained. While obtaining the consent, the couple should be explained how, when, and by whom the procedure will be done and what is the indication to do the procedure. The fetal and maternal risks associated with the procedure should also be discussed in detail. The time required to obtain results, failure to culture cells, and the type of cytogenetic test being performed on the sample obtained should be discussed as well.
  • In Rh-negative women, the need for anti-D to be explained.
  • Proper documentation of the procedure should be done.
  • Ultrasonography before the procedure is done to note the number of fetuses, viability of the fetus, placental location, gestational age, site of cord insertion, and any obvious fetal malformation. 

At the Time of the Procedure

  • Complete aseptic barrier/conditions should be established at the beginning of the procedure by preparing the skin and ultrasound probe.
  • A local anesthetic is not required.
  • Prophylactic antibiotics are not required.

Technique

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.[6] 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.[6] 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.

Complications

In experienced hands, the risk of complications is minimal. Both maternal and fetal complications are experienced with amniocentesis.

  • The fetal loss rate associated with amniocentesis on an average is 0.11%. The loss is 0.56% within 28 days, 0.09% within 42 days.[6]
  • Amniotic fluid leak: 1% to 2%, and usually associated with spontaneous sealing of membranes[6]
  • There is a 2% to 3% risk of vaginal bleeding.
  • An estimated 2.6% risk of fetomaternal hemorrhage.
  • There is minimal chance of introduction of skin bacteria into the amniotic cavity. The risk of chorioamnionitis and uterine infections is less than 0.1%.
  • The procedure increases the risk of preterm, preterm premature rupture of membrane, oligohydramnios.
  • There is a minimal chance of fetal injury, including ocular, cutaneous injuries. The risk of talipoequinovarus is higher with early amniocentesis and increases when there is amniotic fluid leakage.
  • Post-procedure pain and maternal discomfort: Mean pain intensity described is 1.6+/-1.3 when noted on a scale of 0-7.[7]

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.[8]

Clinical Significance

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.[9] Passing the needle through the placenta is slightly associated with an increase in rates of preterm birth.[10]

Enhancing Healthcare Team Outcomes

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. 


References

[1] Pinette MG,Wax J,Blackstone J,Cartin A,McCrann D, Timing of early amniocentesis as a function of membrane fusion. Journal of clinical ultrasound : JCU. 2004 Jan;     [PubMed PMID: 14705171]
[2] Practice Bulletin No. 162 Summary: Prenatal Diagnostic Testing for Genetic Disorders. Obstetrics and gynecology. 2016 May;     [PubMed PMID: 27101119]
[3] ACOG Practice Bulletin No. 88, December 2007. Invasive prenatal testing for aneuploidy. Obstetrics and gynecology. 2007 Dec;     [PubMed PMID: 18055749]
[4] Tabor A,Alfirevic Z, Update on procedure-related risks for prenatal diagnosis techniques. Fetal diagnosis and therapy. 2010;     [PubMed PMID: 20051662]
[5] Mathai M, Amniocentesis. The National medical journal of India. 1992 Jan-Feb;     [PubMed PMID: 1304245]
[6] Ghi T,Sotiriadis A,Calda P,Da Silva Costa F,Raine-Fenning N,Alfirevic Z,McGillivray G, ISUOG Practice Guidelines: invasive procedures for prenatal diagnosis. Ultrasound in obstetrics     [PubMed PMID: 27485589]
[7] Harris A,Monga M,Wicklund CA,Robbins-Furman PJ,Strecker MN,Doyle NM,Mastrobattista J, Clinical correlates of pain with amniocentesis. American journal of obstetrics and gynecology. 2004 Aug;     [PubMed PMID: 15343234]
[8] Jummaat F,Ahmad S,Mohamed Ismail NA, 5-Year review on amniocentesis and its maternal fetal complications. Hormone molecular biology and clinical investigation. 2019 Sep 20;     [PubMed PMID: 31539354]
[9] Kalogiannidis I,Prapa S,Dagklis T,Karkanaki A,Petousis S,Prapas Y,Prapas N, Amniocentesis-related adverse outcomes according to placental location and risk factors for fetal loss after midtrimester amniocentesis. Clinical and experimental obstetrics     [PubMed PMID: 21995155]
[10] Chaksuwat P,Wanapirak C,Piyamongkol W,Sirichotiyakul S,Tongprasert F,Srisupundit K,Luewan S,Traisrisilp K,Jatavan P,Tongsong T, A comparison of pregnancy outcomes after second-trimester amniocentesis between cases with penetration of the placenta and nonpenetration. The journal of maternal-fetal     [PubMed PMID: 32299277]