Amniocentesis

1. DEFINITION

• An invasive prenatal diagnostic procedure involving the transabdominal withdrawal of amniotic fluid from the amniotic sac under ultrasound guidance.
• Target: Amniocytes (fetal skin/urinary tract cells) and supernatant fluid.

2. TIMING

• Ideal Time: 15–20 weeks of gestation.
• Early Amniocentesis (11–14 weeks): Generally avoided due to higher rates of fetal loss, fluid leakage, and Talipes Equinovarus (Clubfoot).
• Late Amniocentesis (>24 weeks): Used for managing polyhydramnios or rarely for fetal lung maturity.

3. INDICATIONS

A. Diagnostic (Genetic/Structural)

• Abnormal Screening: High-risk NIPT/NIPS, abnormal Combined/Quadruple test.
• Ultrasound Anomalies: Detection of soft markers or structural defects.
• Previous History: Previous child with chromosomal abnormality or metabolic disorder.
• Parental Carrier Status: Balanced translocation or autosomal recessive trait (e.g., Thalassemia, SMA).
• Biochemical: Elevated maternal serum AFP (to test amniotic AFP + Acetylcholinesterase for NTDs).

B. Infectious (Fetal Infection)

• PCR analysis of amniotic fluid for CMV, Toxoplasmosis, Parvovirus B19, or Zika virus.
• Note: CMV testing is best done >21 weeks and >6 weeks post-maternal infection for sensitivity.

C. Therapeutic

• Amnioreduction: Relief of severe symptomatic Polyhydramnios (e.g., in Twin-Twin Transfusion Syndrome).

4. PROCEDURE (Technique)

1. Pre-procedure: Ultrasound to map placental position, fetal heart rate (FHR), and fluid pockets.
2. Asepsis: Povidone-iodine cleaning of the maternal abdomen.
3. Needle Insertion:
   • Continuous Ultrasound Guidance is mandatory.
   • 20–22 Gauge spinal needle inserted transabdominally into the deepest amniotic pool, avoiding the placenta and fetus.
4. Aspiration:
   • First 1–2 mL discarded (to avoid maternal cell contamination).
   • 15–20 mL of fluid aspirated for analysis.
5. Post-procedure: Check FHR immediately. Anti-D Immunoglobulin administered if mother is Rh-negative.

5. LABORATORY ANALYSIS

• Cytogenetics:
  • FISH: Rapid result (24-48 hrs) for trisomies 13, 18, 21, X, Y.
  • Karyotype: Gold standard for aneuploidy and large rearrangements (10-14 days culture time).
  • Chromosomal Microarray (CMA): Preferred first-line test if structural anomalies are present (detects microdeletions).
• Biochemical:
  • Alpha-Fetoprotein (AFP) + Acetylcholinesterase (AChE): Diagnostic for Neural Tube Defects (Open Spina Bifida).
  • Enzyme Assays: For Inborn Errors of Metabolism (Tay-Sachs, Gaucher).
• Lung Maturity (Historical): L/S ratio (Lecithin/Sphingomyelin) >2 indicates maturity (rarely used now).

6. COMPLICATIONS

• Fetal Loss: Risk is 0.1% – 0.3% (1 in 300–500) in experienced hands (lower than historical 0.5%).
• Leakage of Fluid: 1–2% (usually resolves spontaneously).
• Vaginal Bleeding: Spotting occurs in ~1%.
• Chorioamnionitis: Risk <0.1% with proper asepsis.
• Needle Injury: Direct trauma to fetus (rare under USG guidance).
• Rh Isoimmunization: Risk minimized by prophylactic Anti-D.

7. ADVANTAGES OVER CVS (Chorionic Villus Sampling)

• Can test for NTDs (AFP levels) – CVS cannot.
• Lower risk of Mosaicism (Confirmed placental mosaicism is ~1% in CVS; rare in amnio).
• Ideally suited for detailed anatomy scan timing (18–20 weeks).