Prenatal Genetic Testing & Chromosomal Assessment
From first trimester combined screening to NIPT and invasive diagnostic testing — a complete, evidence-based approach to fetal genetic risk assessment at Balaji Horizon.
Understanding Your Options — Screening vs Diagnosis
The single most important distinction in prenatal genetics: Screening tests give a probability. Diagnostic tests give a definitive answer. They are not interchangeable — a positive screening result requires confirmation by a diagnostic test before any irreversible decision is made.
| Type | Tests | What It Tells You | Is It Diagnostic? |
|---|---|---|---|
| Screening | NT scan, Combined Test, NIPT, Quad Test | Risk category / probability ratio | No — identifies high-risk group only |
| Diagnostic | CVS, Amniocentesis, Cordocentesis | Definitive chromosomal result | Yes — 99.9% accuracy |
Non-Invasive Prenatal Screening Options
Combined First Trimester Screening
11–13+6 Weeks
Components: NT measurement + free β-hCG + PAPP-A + maternal age
Down syndrome detection rate: ~90–95% at 5% false positive rate
Output: Individual risk ratio (e.g., 1:150 for Trisomy 21). High-risk (>1:150): NIPT or invasive testing offered.
NIPT — Non-Invasive Prenatal Testing
From 10–12 Weeks
Principle: Cell-free fetal DNA in maternal blood analysed for chromosomal imbalances
Conditions screened: Trisomy 21, 18, 13; sex chromosome aneuploidies; microdeletions (extended panels)
Trisomy 21 sensitivity: >99% with >99.9% specificity
Critical Limitations
NIPT is a screening test — a positive result requires amniocentesis confirmation. Failure rate ~1–3% (insufficient fetal fraction, often in high BMI). Does not screen for structural anomalies. Confined placental mosaicism can cause false positives.
Second Trimester Quadruple Test
15–20 Weeks
Components: AFP + hCG + uE3 + inhibin A + maternal age
Detection rate: ~75–80% for Down syndrome. Used when first trimester screening was missed. Less accurate than combined T1 screening — not the preferred test when first trimester window is available.
NIPT vs Amniocentesis — Understanding the Choice
| Factor | NIPT | Amniocentesis |
|---|---|---|
| Invasive? | No — blood test only | Yes — needle into the uterus under ultrasound |
| Risk to pregnancy | None from the test itself | ~0.1–0.3% procedure-related loss risk |
| Timing | From 10–12 weeks | 15–18 weeks |
| Diagnostic? | No — screening only | Yes — definitive chromosomal result |
| What it detects | Chromosomal imbalances, selected microdeletions | Full karyotype, microarray, single-gene disorders |
| Failure rate | ~1–3% (low fetal fraction) | Rarely fails |
| Turnaround | 7–14 days | 48–72 hrs (rapid FISH); 2–3 weeks (full karyotype) |
Clinical Decision Framework
- High-risk T1 combined screen + anxious about amniocentesis → NIPT first, then amnio if positive
- Abnormal NT (≥3.5 mm) + structural anomaly on scan → proceed directly to amniocentesis (NIPT insufficient)
- Advanced maternal age alone, no abnormal scan → NIPT appropriate first-line
- Any structural anomaly on ultrasound → amniocentesis preferred over NIPT
- Positive NIPT result (any) → amniocentesis required before any irreversible decision
Frequently Asked Questions
Evidence-based answers about prenatal genetic testing.
Is NIPT available at Balaji Horizon?
Yes. We coordinate NIPT sample collection with pre-test counselling. Blood is taken at our facility and sent to an accredited laboratory. Results are reviewed and explained by Dr. Mayank Chaudhary in the clinical context of your scan and history. We do not offer NIPT in isolation without counselling.
Can NIPT diagnose Down syndrome with certainty?
No. NIPT is a screening test with very high sensitivity and specificity — but it is not diagnostic. A positive NIPT result must always be confirmed by amniocentesis or CVS before any clinical or personal decision is made. A negative NIPT result significantly reduces risk but cannot completely exclude Down syndrome.
What if I decline genetic testing?
That is entirely your right. Prenatal genetic testing is offered — never mandated. The decision belongs to the parents, informed by the information provided. We provide balanced information and support whatever choice is made. No pressure, no judgment.
Does a normal NT scan mean my baby definitely doesn’t have Down syndrome?
A normal NT reduces risk significantly, but does not eliminate it. Accurate risk calculation requires NT + blood markers (free β-hCG + PAPP-A) + maternal age together — this is the Combined First Trimester Screening. The final risk ratio guides whether further testing is appropriate. NT alone is not the full screening test.
At what age does Down syndrome risk increase significantly?
Risk increases with maternal age — approximately 1:1,500 at age 25, 1:270 at age 35, and 1:100 at age 40. However, because younger women have more pregnancies overall, the majority of Down syndrome births statistically occur in women under 35. This is why screening is recommended regardless of maternal age.
What is chromosomal microarray and when is it recommended?
Chromosomal microarray (CMA) detects submicroscopic deletions and duplications — copy number variants (CNVs) — not visible on standard karyotype. It is recommended when ultrasound identifies structural anomalies alongside a normal standard chromosome result, or when more detailed genetic analysis is required after amniocentesis. It expands the diagnostic resolution beyond what conventional karyotyping provides.
Book a Prenatal Genetics Consultation
For NIPT coordination, high-risk screening counselling, or pre-amniocentesis consultation. Expert fetal medicine team at Balaji Horizon, Ahmedabad.
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