Introduction
Amniocentesis is an invasive prenatal diagnostic test in which a small amount of amniotic fluid is withdrawn from the uterine cavity using a thin needle under continuous ultrasound guidance. The fluid contains fetal cells and biochemicals, which can be analyzed to diagnose genetic and chromosomal disorders, fetal infections, and other conditions. It is primarily performed between 15–20 weeks of gestation.
Indications
Detection of Chromosomal Abnormalities: Amniocentesis is commonly used to detect chromosomal abnormalities such as Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). These conditions are associated with significant developmental and health challenges.
Diagnosis of Single-Gene Disorders: The procedure can diagnose single-gene disorders like thalassemia, cystic fibrosis, and sickle cell anemia. These disorders are caused by mutations in specific genes and can have serious health implications for the affected fetus.
Confirmation of Abnormal Screening Results: When non-invasive prenatal testing (NIPT), biochemical marker screening, or ultrasound examinations reveal anomalies, amniocentesis is often performed to confirm or rule out specific genetic or chromosomal conditions.
Evaluation of Fetal Infections: Amniocentesis can be used to evaluate fetal infections, such as cytomegalovirus (CMV) and toxoplasmosis. Detecting these infections early can help guide treatment decisions and manage potential complications.
Therapeutic Use in Polyhydramnios (Amnioreduction): In cases of severe polyhydramnios (excessive amniotic fluid), amniocentesis can be used therapeutically to remove excess fluid, relieving maternal discomfort and reducing the risk of preterm labor.
Procedure
Pre-Procedure Genetic Counseling and Consent: Before the procedure, patients receive genetic counseling to understand the risks, benefits, and limitations of amniocentesis. Informed consent is obtained, ensuring the patient is fully aware of the process.
Ultrasound Evaluation: An ultrasound examination is performed to locate the placenta, fetus, and a suitable pocket of amniotic fluid. This helps guide the needle insertion and minimize the risk of injury to the fetus or placenta.
Needle Insertion: A thin needle is inserted through the abdominal wall into the amniotic sac under continuous ultrasound guidance. This ensures precise placement and avoids contact with the fetus.
Amniotic Fluid Aspiration: Approximately 15–20 mL of amniotic fluid is aspirated. The initial sample is typically discarded to avoid contamination with maternal cells.
Post-Procedure Ultrasound: After the fluid is collected, another ultrasound is performed to confirm fetal well-being and assess for any immediate complications.
Laboratory Analysis
Rapid Aneuploidy Testing: Rapid aneuploidy testing using quantitative fluorescence polymerase chain reaction (QF-PCR) or fluorescence in situ hybridization (FISH) can provide results within 2–3 days. This testing focuses on common chromosomal abnormalities such as Down syndrome, Edwards syndrome, and Patau syndrome.
Karyotyping or Chromosomal Microarray: Karyotyping or chromosomal microarray analysis provides a detailed genetic analysis, identifying a wide range of chromosomal abnormalities. Results typically take 10–14 days.
DNA-Based Testing: DNA-based testing is used to detect specific single-gene disorders. This involves analyzing the fetal DNA for mutations associated with conditions like cystic fibrosis or thalassemia.
PCR for Infectious Agents: Polymerase chain reaction (PCR) is used to detect infectious agents, such as CMV or toxoplasmosis, when clinically indicated. This helps in the diagnosis and management of fetal infections.
Although generally safe in experienced hands, amniocentesis carries certain risks:
1. Cramping and mild pain.
2. Vaginal spotting or fluid leakage.
3. Procedure-related miscarriage risk: approximately 0.1–0.3%.
4. Rare infection or needle injury.
After-care
Patients may experience mild cramps for a day and should avoid heavy work for 24–48 hours. They should immediately report persistent pain, fever, vaginal bleeding, fluid leakage, or reduced fetal movements (later in pregnancy).
Alternatives
1. Chorionic Villus Sampling (CVS) –CVS is performed at 10–13 weeks of gestation and involves sampling placental tissue. However, CVS cannot detect neural tube defects.
2. Non-invasive Prenatal Testing (NIPT) –NIPT is a screening test that analyzes fetal DNA in the maternal blood. While NIPT is highly accurate, it is a screening test and requires confirmation by CVS or amniocentesis if high-risk results are obtained.
FAQs
Q1. Does amniocentesis hurt?
Ans: Most women report mild discomfort similar to period cramps.
Q2. Can amniocentesis detect all birth defects?
Ans: No. It detects many chromosomal and known genetic disorders but not all structural anomalies.
Q3. How accurate is it?
Ans: Diagnostic accuracy is very high (>99% for common aneuploidies).
Q4. How long does it take to get results?
Ans: Rapid tests in 2–3 days; full karyotype or microarray in 10–14 days.
Worksheet (Short Questions)
1. Define amniocentesis.
2. State the ideal gestational age for performing amniocentesis.
3. Mention three clinical indications.
4. List complications of the procedure.
5. Differentiate between amniocentesis, CVS, and NIPT.
MCQs
1) Amniocentesis is usually performed at:
A. 8–10 weeks
B. 10–13 weeks
C. 15–20 weeks
D. After 36 weeks
Answer: C
2) Which test provides rapid results for aneuploidies?
A. Karyotype
B. FISH/QF-PCR
C. Chromosomal microarray
D. DNA sequencing
Answer: B
3) Risk of miscarriage after amniocentesis is approximately:
A. 5–10%
B. 1–2%
C. 0.1–0.3%
D. 0%
Answer: C
References
1. Williams Obstetrics, 26th Edition, McGraw-Hill Education.
2. Bhatla N. Textbook of Obstetrics, 8th Edition, Jaypee Brothers.
3. Dutta DC. Textbook of Obstetrics, 9th Edition, Jaypee Brothers.
4. Cunningham FG et al. Williams Manual of Obstetrics.
5. ACOG Practice Bulletin: Prenatal Diagnostic Testing for Genetic Disorders.
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