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.

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.

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).

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.

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

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

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).

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.

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).