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AN81.1-3 | Prenatal Diagnosis — Self-Directed Learning
CLINICAL SCENARIO
A 38-year-old primigravida from Chennai presents at 14 weeks of gestation. She underwent IVF after years of infertility. Her first-trimester combined screening (nuchal translucency + PAPP-A + free beta-hCG) shows a 1-in-100 risk of trisomy 21.
The obstetrician counsels: "Your risk is high based on the screening test. We can offer you a diagnostic test to confirm or exclude Down syndrome. Options include amniocentesis or CVS. Both carry a small risk of miscarriage."
The patient asks: "What exactly is Down syndrome? How would you test for it? Can you test without a needle? What are the risks?"
What are the prenatal diagnostic options available at 14 weeks? What are the indications, procedure, and risks of amniocentesis vs CVS? Are there non-invasive alternatives?
WHY THIS MATTERS
Prenatal diagnosis is a critical topic in Indian obstetric and ethical practice:
- Advanced maternal age trend — increasing average age of first pregnancy in urban India → higher risk of chromosomal anomalies (trisomy 21, 18, 13)
- PCPNDT Act 1994 (Pre-Conception and Pre-Natal Diagnostic Techniques Act) — prohibits sex determination for the purpose of sex-selective abortion; regulates prenatal diagnostic laboratories in India; violation = criminal offence
- NIPT (Non-Invasive Prenatal Testing) — cell-free fetal DNA in maternal blood; now widely available in Indian private hospitals; screening (not diagnostic) for common trisomies
- NTD screening — maternal serum AFP + ultrasound for neural tube defects; part of triple/quadruple screen offered in India
- Beta-thalassaemia carrier screening — high prevalence in India; prenatal diagnosis by CVS or amniocentesis with DNA analysis prevents affected births
- NMC 2024 CBME — AN81.1–81.3 are core competencies; knowledge of amniocentesis and CVS is assessed in theory examinations and clinical OSPE
RECALL
Before we begin, recall:
- Chromosome number: Human somatic cells = 46 chromosomes (23 pairs); gametes = 23
- Trisomy 21 (Down syndrome): 47 chromosomes (+21); most common chromosomal cause of intellectual disability; risk increases with maternal age (1:1,500 at 20 yrs → 1:30 at 45 yrs)
- Amniotic fluid: Contains exfoliated fetal cells; accessible by needle through the anterior abdominal wall
- Chorionic villi: Fetal tissue (same DNA as the fetus); accessible transcervically or transabdominally in early pregnancy
Part 1: Overview of Prenatal Diagnosis Methods (AN81.1)
Classification of Prenatal Diagnostic Methods
A. NON-INVASIVE METHODS
1. Ultrasonography (USG)
• Most widely used prenatal diagnostic tool worldwide and in India
• 1st trimester scan (11–13+6 weeks):
- Dating scan: confirms gestational age (crown-rump length)
- Nuchal translucency (NT) measurement: Fluid-filled space at back of fetal neck; NT >3.5 mm → increased risk of trisomies and cardiac defects
- Fetal nasal bone presence/absence
• Anomaly scan (18–20 weeks):
- Detailed anatomy survey: brain, spine, face, heart, abdomen, limbs, genitalia
- Detects: NTDs, cardiac defects, renal anomalies, skeletal dysplasias, omphalocele, gastroschisis
- PCPNDT Act: sex must NOT be disclosed to family during this scan in India
• Fetal echocardiography (22–24 weeks): For cardiac anomalies
2. Maternal Serum Biochemical Screening
| Trimester | Test | Markers | Detects |
|---|---|---|---|
| 1st (10–13 wks) | Combined screening | NT + PAPP-A + free β-hCG | Trisomy 21, 18, 13 |
| 2nd (15–20 wks) | Triple screen | AFP + hCG + uE3 | Trisomy 21, NTDs |
| 2nd (15–20 wks) | Quadruple screen | AFP + hCG + uE3 + inhibin A | Trisomy 21 (↑sensitivity) |
Marker patterns:
| Condition | AFP | hCG | uE3 | Inhibin A |
|---|---|---|---|---|
| Down syndrome (T21) | ↓ | ↑ | ↓ | ↑ |
| Trisomy 18 | ↓ | ↓ | ↓ | — |
| Open NTD | ↑↑ | — | — | — |
3. Non-Invasive Prenatal Testing (NIPT / cfDNA testing)
• Cell-free fetal DNA (cffDNA) from placental trophoblast cells enters maternal bloodstream from 10 weeks
• Maternal blood sample analysed by next-generation sequencing
• Detects: Trisomies 21, 18, 13; sex chromosome aneuploidies; fetal sex
• Sensitivity/specificity: >99% for T21 — the best screening test available
• Important: NIPT is a SCREENING test, NOT diagnostic — a positive NIPT must be confirmed by invasive testing (amniocentesis or CVS) with karyotyping
• Cost: ~₹15,000–20,000 in India (not universally available in government hospitals)
B. SEMI-INVASIVE METHODS
4. Preimplantation Genetic Testing (PGT)
• In IVF cycle: 1–2 cells biopsied from 8-cell embryo or blastocyst before implantation
• DNA tested for specific mutations (PGT-M) or aneuploidy (PGT-A)
• Selected unaffected embryos transferred
• Used in India for: beta-thalassaemia major, Huntington's disease, familial BRCA mutations
C. INVASIVE METHODS (Diagnostic)
• Amniocentesis (2nd trimester, 15–20 weeks) → see Part 2
• Chorionic Villus Sampling (CVS) (1st trimester, 10–13 weeks) → see Part 3
• Cordocentesis (PUBS) — 18 weeks onwards; fetal blood from umbilical cord; used for fetal blood disorders, rapid karyotype
Part 2: Amniocentesis — Indications, Procedure, Risks (AN81.2)
Amniocentesis
Definition: Transabdominal aspiration of amniotic fluid from the amniotic cavity under ultrasound guidance.
Optimal timing: 15–20 weeks of gestation
• Before 15 weeks = "early amniocentesis" (higher risk; not recommended)
• After 20 weeks = delayed diagnosis
Indications:
1. Chromosomal analysis (most common):
- Advanced maternal age (≥35 years in India)
- Abnormal first-trimester combined screening result
- Positive NIPT result (for confirmation)
- Previous child with chromosomal anomaly
- Parental chromosomal rearrangement (balanced translocation)
2. Biochemical analysis:
- Neural tube defects (elevated AFP in amniotic fluid)
- Inborn errors of metabolism (enzyme assay on fetal cells)
3. DNA analysis:
- Beta-thalassaemia, sickle cell disease, cystic fibrosis, Duchenne muscular dystrophy
4. Fetal lung maturity (in late pregnancy, <34 weeks before planned delivery):
- Lecithin/sphingomyelin (L/S) ratio ≥2 = mature lungs
5. Therapeutic amniocentesis — relief of severe polyhydramnios
Procedure:
1. Pre-procedure ultrasound: confirm gestation, placental site, fetal position, adequate fluid pool
2. Aseptic technique; transabdominal approach
3. 22-gauge spinal needle inserted under continuous USG guidance, avoiding the placenta and fetal parts
4. First 1–2 mL discarded (to avoid maternal cell contamination)
5. 15–20 mL of amniotic fluid aspirated and sent to cytogenetics laboratory
6. Cells cultured for 10–14 days → karyotype by conventional G-banding OR FISH (Fluorescence In Situ Hybridisation) for rapid result in 48–72 hours for common aneuploidies
7. Post-procedure: USS to confirm fetal heartbeat; anti-D prophylaxis if mother is Rh-negative
Results from amniotic fluid:
| Sample | What is tested |
|---|---|
| Fetal cells (cultured) | Karyotype (chromosomes), DNA analysis |
| Supernatant fluid | AFP (NTD screening), biochemical markers |
| Uncultured cells | FISH for rapid aneuploidy detection |
Disadvantages / Risks:
1. Procedure-related pregnancy loss: 0.1–0.5% (1 in 200–1,000); this risk must be weighed against the background risk of the condition being tested
2. Late result: Cell culture takes 2 weeks → diagnosis at 17–18 weeks; termination at this stage carries higher morbidity
3. Needle injury to fetus: Rare (1 in 1,000)
4. Chorioamnionitis: Infection of amniotic cavity; rare but serious
5. Amniotic fluid leakage: Transient in 1–2%; usually self-sealing
6. Fetal club foot: Associated with early amniocentesis (<15 weeks) — extra caution advised
7. False-positive / false-negative: Culture failure (~0.5%); maternal cell contamination can give false XX result in male fetus
8. Psychological stress: Waiting 2 weeks for results causes significant anxiety
Part 3: Chorionic Villus Sampling (CVS) — Indications, Procedure, Risks (AN81.3)
Chorionic Villus Sampling (CVS)
Definition: Sampling of chorionic villi (placental trophoblastic tissue) from the developing placenta for cytogenetic, biochemical, or DNA analysis.
Optimal timing: 10–13+6 weeks of gestation (1st trimester)
• Before 10 weeks = associated with transverse limb defects (limb reduction defects)
Principle: Chorionic villi have the same chromosomal/DNA composition as the fetus → direct diagnosis from placental tissue.
Routes of access (two techniques):
| Route | Technique | When preferred |
|---|---|---|
| Transcervical | Flexible catheter through cervix under USG guidance | Posterior implantation, easier cervical access |
| Transabdominal | 20-gauge needle through anterior abdominal wall under USG guidance | Anterior/fundal implantation; preferred in most Indian centres |
Procedure:
1. USG confirms gestational age, placental location, and access route
2. Aseptic preparation; transabdominal or transcervical approach
3. 10–25 mg of chorionic villi aspirated using a syringe under continuous USG guidance
4. Villi identified under dissecting microscope to confirm placental tissue (not decidua)
5. Sent in culture medium to cytogenetics
6. Direct preparation: Results in 24–48 hours (mitoses in cytotrophoblast cells)
7. Cultured cells: Results in 7–10 days (more accurate; cultured mesenchymal cells)
Advantages over amniocentesis:
| Feature | CVS | Amniocentesis |
|---|---|---|
| Timing | 10–13 weeks (1st trimester) | 15–20 weeks (2nd trimester) |
| Result time | 48–72 hours (direct prep) | 10–14 days |
| Termination if abnormal | Medical termination at 12–13 weeks (safer, less traumatic) | D&E at 17–18 weeks (more morbidity) |
| Sample | Adequate amount; rich in DNA | Limited viable cells |
Disadvantages / Risks:
1. Procedure-related pregnancy loss: 1–2% (higher than amniocentesis: 0.1–0.5%)
2. Limb reduction defects: Rare (1 in 3,000) if performed <10 weeks → avoid before 10 weeks; mechanism = vascular disruption
3. Confined placental mosaicism (CPM): The chorionic villi may show a chromosomal anomaly NOT present in the fetus → false-positive result; requires amniocentesis to confirm
- CPM occurs in ~1–2% of CVS cases
- Due to mitotic error in trophoblast cells that was corrected in the embryo
4. Rh sensitisation: Anti-D must be given to Rh-negative women
5. Cannot measure AFP (no amniotic fluid) → cannot diagnose open NTDs by CVS alone; USG needed
6. Infection: Chorioamnionitis (rare)
7. No fetal lung maturity assessment (not applicable in 1st trimester)
Comparison Summary:
| Feature | NIPT (cfDNA) | CVS | Amniocentesis |
|---|---|---|---|
| Type | Screening | Diagnostic | Diagnostic |
| Timing | 10+ weeks | 10–13 wks | 15–20 wks |
| Sensitivity (T21) | >99% | ~100% | ~100% |
| Miscarriage risk | 0% | 1–2% | 0.1–0.5% |
| Result time | 10–14 days | 48–72 hrs (direct) | 10–14 days |
| Can diagnose NTDs? | No | No | Yes (AFP) |
| Detects mosaicism? | Limited | CPM issue | Better |
PCPNDT Act implications:
All prenatal diagnostic procedures in India must be performed only at registered centres, with written consent, and ONLY for specified medical indications. The result of fetal sex determination must NEVER be communicated to the family.
SELF-CHECK — Self-Check: Prenatal Diagnosis
A 36-year-old woman has a positive NIPT result showing high risk for trisomy 21. The next most appropriate step is:
A. Proceed directly to termination of pregnancy
B. Offer confirmatory invasive diagnostic testing (amniocentesis or CVS) with karyotyping
C. Repeat the NIPT test for confirmation
D. No further action needed as NIPT sensitivity is >99%
Reveal Answer
Answer: B. Offer confirmatory invasive diagnostic testing (amniocentesis or CVS) with karyotyping
Chorionic villus sampling at 9 weeks of gestation carries a risk of causing which specific fetal malformation?
A. Neural tube defects
B. Cardiac septal defects
C. Transverse limb reduction defects
D. Renal agenesis
Reveal Answer
Answer: C. Transverse limb reduction defects
A 17-week amniocentesis shows 46XY on G-banded karyotype. However, the obstetrician suspects maternal cell contamination. Which supplementary test would best confirm whether the karyotype is truly fetal?
A. Maternal serum AFP
B. Fetal echocardiography
C. Repeat amniocentesis immediately
D. STR (short tandem repeat) profiling comparing maternal and amniotic fluid DNA
Reveal Answer
Answer: D. STR (short tandem repeat) profiling comparing maternal and amniotic fluid DNA
CLINICAL PEARL
Prenatal Diagnosis in the Indian Context — Ethical and Legal Framework
PCPNDT Act 1994 (Pre-Conception and Pre-Natal Diagnostic Techniques Act):
• Enacted to prevent sex-selective abortion in India (son preference → female foeticide)
• Prohibits:
- Conducting PND tests without registration
- Sex selection before or after conception
- Communicating fetal sex to the patient or family
- Advertising PND facilities
• Requires:
- All centres performing USG, amniocentesis, CVS, NIPT must be registered under PCPNDT
- Mandatory consent forms stating the test is NOT for sex determination
- Record of all pregnant women scanned
- Radiologists must not tilt screen to show genitalia
• Penalties: 3–5 years imprisonment; fine ₹10,000–50,000; loss of registration
Triple Screen Pattern for Down Syndrome (T21):
• AFP ↓ + hCG ↑ + uE3 ↓
• Mnemonic: "AhA" — AFP down, hCG UP, uE3 down
Alpha-Fetoprotein (AFP) — Dual Role:
• ELEVATED in: Open NTDs (anencephaly, myelomeningocele), ventral wall defects (gastroschisis, omphalocele), multiple pregnancy, fetal demise
• DECREASED in: Down syndrome, trisomy 18
• Remember: AFP is produced by the fetal yolk sac and liver; enters amniotic fluid via fetal urine and transudation from skin (only until 14 weeks, when skin keratinises)
REFLECT
Return to the 38-year-old primigravida from the hook case:
- Her first-trimester combined screen showed 1-in-100 risk for T21. Is this a diagnostic result or a screening result? What are the next steps?
- The patient is 14 weeks. She opts for amniocentesis. Why is 14 weeks suboptimal for amniocentesis? What is the recommended timing?
- If CVS had been offered at 11 weeks and the result showed 47,XX+21 — trisomy 21 — the patient decides to continue the pregnancy. Which services and support should be offered? How does the PCPNDT Act affect your communication?
- The result from CVS shows 47,XY+21 in trophoblast cells but 46,XY in cultured mesenchymal cells. What is this phenomenon called, and what further test is needed?
Discussion: (1) Screening result only — must be confirmed by amniocentesis/CVS karyotype. (2) Amniocentesis before 15 weeks = higher miscarriage risk and associated with fetal club foot; optimal is 15–20 weeks. (3) Offer continued support, counselling, and refer to paediatric developmental services; disclosure of sex is prohibited by PCPNDT Act regardless of indication. (4) Confined placental mosaicism (CPM) — trophoblast cells show trisomy but fetal cells (mesenchymal) are normal; this is a false-positive CVS result; amniocentesis must be done to obtain true fetal karyotype.
KEY TAKEAWAYS
Key Takeaways — Prenatal Diagnosis (AN81.1–81.3)
Methods Overview (AN81.1):
• Non-invasive: USG (anomaly scan at 18–20 wks), maternal serum biochemistry (triple/quadruple screen), NIPT (cfDNA, screening only)
• Invasive (diagnostic): CVS (10–13 wks), amniocentesis (15–20 wks), cordocentesis (≥18 wks)
Amniocentesis (AN81.2):
• Timing: 15–20 weeks; 15–20 mL fluid
• Indications: Advanced maternal age, abnormal screen, DNA diagnosis, AFP (NTDs)
• Risks: 0.1–0.5% miscarriage, culture takes 2 weeks, late result
CVS (AN81.3):
• Timing: 10–13 weeks; transcervical or transabdominal
• Advantage over amniocentesis: earlier diagnosis, faster result (48–72 hrs), 1st trimester termination if needed
• Risks: 1–2% miscarriage (higher than amnio), limb defects if <10 wks, confined placental mosaicism (CPM — false positive)
• Cannot assess AFP or fetal lung maturity
India-specific:
• PCPNDT Act 1994 — prohibits sex determination; all PND centres must be registered
• NIPT widely available in private hospitals (~₹15,000–20,000)
• Beta-thalassaemia carrier screening — high priority in Indian prenatal care