हिंदी
Tamil Nadu Board of Secondary EducationHSC Science Class 12
पाठ्यपुस्तक उत्तर१०९५२
अवधारणा स्पष्टीकरण२००
पाठ्यक्रम

Amniocentesis

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Estimated time: 18 minutes
CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Definition: Amniocentesis

Amniocentesis is a technique used to diagnose fetal abnormalities by drawing a sample of amniotic fluid by a hypodermic needle inserted through the mother’s abdomen into the uterus

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Introduction

Amniocentesis is a foetal prenatal diagnostic technique in which a sample of amniotic fluid, containing shed foetal cells, is withdrawn from the amniotic sac using a hypodermic needle inserted through the mother's abdomen and uterine wall under ultrasound guidance, and is then subjected to chromosomal, genetic, and biochemical analysis to detect foetal abnormalities.

Amniocentesis

Key term to remember: Amniotic fluid contains free-floating foetal cells (shed from skin, liver, and placenta), urine, and proteins, including alpha-fetoprotein - all of which carry complete foetal DNA.

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Indications - Who Needs This Test?

Amniocentesis is not a routine prenatal test. It is recommended in specific high-risk situations:

  • Mother is 35 years of age or older at the time of delivery (higher risk of chromosomal non-disjunction)
  • Previous pregnancy affected by a chromosomal or genetic disorder
  • Abnormal results from earlier screening tests (e.g., triple/quadruple marker screen, nuchal translucency scan)
  • Family history of a known genetic disorder (e.g., cystic fibrosis, sickle cell anaemia, haemophilia)
  • Abnormal ultrasound findings (structural malformations detected)
  • Maternal carrier status for X-linked disorders (e.g., Duchenne Muscular Dystrophy)
  • Assessment of foetal lung maturity in the third trimester when early delivery is being considered​
  • Detection of Rh incompatibility severity
CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Procedure

Optimal Timing: 14th–20th week of pregnancy (most commonly 15th–16th week). Too early = higher miscarriage risk.

Step-by-Step Procedure:

Step 1: Pre-Procedure Assessment
Patient lies down; BP, heart rate, and breathing are measured.

Step 2: Ultrasound Scan
Locates foetus, placenta, umbilical cord, and amniotic fluid pocket to guide safe needle placement.

Step 3: Preparation of the Abdomen
Cleaned with antiseptic; sterile gel applied; local anaesthesia given if needed.

Step 4: Needle Insertion
Long, hollow needle inserted through abdominal wall → uterine wall → amniotic sac, under live ultrasound guidance.

Step 5: Fluid Withdrawal
~15–20 mL of amniotic fluid collected into a sterile, light-protected container.

Step 6: Needle Removal & Post-Procedure Monitoring
Needle withdrawn; foetal heart rate rechecked via ultrasound; patient advised rest for 24 hours, no heavy lifting.

Step 7: Laboratory Analysis
Foetal cells separated → cultured for several days → karyotyping under microscope + biochemical assays performed.

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Conditions detectable by Amniocentesis

Chromosomal Abnormalities:

Condition Chromosomal Basis Key Feature
Down Syndrome (Trisomy 21) Extra chromosome 21 (47 chromosomes) Intellectual disability, characteristic facial features
Edward's Syndrome (Trisomy 18) Extra chromosome 18 Severe developmental defects
Patau's Syndrome (Trisomy 13) Extra chromosome 13 Multiple organ malformations
Turner Syndrome Missing X chromosome (45,X) Short stature, infertility in females
Klinefelter Syndrome Extra X in males (47,XXY) Tall stature, infertility in males

Neural Tube Defects (NTDs):

Condition Marker Used
Spina Bifida Elevated AFP + AChE in amniotic fluid
Anencephaly Elevated AFP levels

Genetic / Metabolic Disorders:

Condition Type
Cystic Fibrosis Single-gene (CFTR mutation)
Sickle Cell Anaemia Haemoglobinopathy
Fragile X Syndrome Methylation study on amniotic fluid
Inborn Errors of Metabolism Biochemical testing
Phenylketonuria (PKU) Metabolic enzyme deficiency

Other Uses:

  • Fetal lung maturity assessment before early delivery
  • Detection of Rh incompatibility/alloimmunization severity
  • Fetal infections diagnosis
CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Risks and Limitations

Risks:

  • Miscarriage: risk ≈ 0.1% to 0.3% (approximately 1 in 500 to 1 in 1,000 procedures in modern practice)
  • Temporary uterine cramping
  • Amniotic fluid leakage (~1–2% cases, usually self-limiting)
  • Infection (chorioamnionitis): very rare (<0.1%)
  • Preterm labour: rare
  • Rh sensitisation: risk if mother is Rh-negative (prevented by anti-D injection)

Limitations:

  • Results take 1–2 weeks (conventional karyotype culture)
  • Cannot detect all genetic conditions or structural malformations
  • Emotional burden: late detection (17–20 weeks) makes decisions harder
  • Not 100% definitive for all disorders
CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Legal and Ethical Status in India

The PCPNDT Act, 1994:

The Pre-Conception and Pre-Natal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994

Key Provisions:

Amniocentesis and other prenatal diagnostic techniques are legally permitted ONLY for detecting:

  • Chromosomal abnormalities
  • Metabolic disorders
  • Genetic conditions
  • Haemoglobinopathies
  • Sex-linked disorders
  • Certain congenital malformations

It is ILLEGAL to use amniocentesis for sex determination of the fetus.

No clinic, lab, or genetic counselling centre can communicate the sex of the fetus to the patient or family (verbally, in writing, or symbolically).

All diagnostic centres must be registered under the Act.

Punishment: Up to 3 years imprisonment and a ₹10,000 fine for first-time offenders.

Even advertising for prenatal sex determination is a punishable offence.

Why the ban? India's skewed sex ratio (female foeticide) led to this legislation to protect the girl child and prevent gender-based discrimination before birth.

CBSE: Class 12
Maharashtra State Board: Class 12
CISCE: Class 12

Key Points: Amniocentesis

  • Amniocentesis is a prenatal diagnostic technique in which amniotic fluid is withdrawn from the uterus to detect fetal genetic and developmental abnormalities.
  • It is usually performed before the 15th week of pregnancy, especially in women above 35 years or those at risk of genetic disorders.
  • The test helps identify chromosomal abnormalities such as Down syndrome, trisomy 13, trisomy 18, neural tube defects, and certain metabolic disorders.
  • Although medically beneficial, misuse of amniocentesis for sex determination is illegal and ethically unacceptable.
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