Prenatal diagnosis of chromosomal trisomies, predominantly Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome), aims to provide expectant parents with accurate information regarding fetal genetic health to facilitate informed reproductive choices and perinatal management.
Indications for Prenatal Screening and Diagnosis
- Advanced maternal age, typically 35 years or older at the time of delivery.
- Previous child affected by a chromosomal abnormality.
- Either parent identified as a carrier of a balanced structural rearrangement, such as a Robertsonian translocation.
- Abnormal findings detected on routine fetal ultrasonography.
- Positive results obtained from non-invasive maternal serum screening tests.
Non-Invasive Screening Strategies
First Trimester Screening
- Timing: Performed between 11 and 13+6 weeks of gestation.
- Ultrasonography Markers: Increased Nuchal Translucency (NT), typically >3.0 mm or >95th percentile, and absent nasal bone are robust markers for aneuploidy.
- Biochemical Markers:
- Pregnancy-Associated Plasma Protein A (PAPP-A) is significantly decreased in Trisomies 21, 18, and 13.
- Free beta-hCG is elevated in Trisomy 21, but decreased in Trisomies 18 and 13.
Second Trimester Screening
- Timing: Performed between 15 and 20 weeks of gestation.
- Biochemical Markers: Utilizes Triple Marker Test (AFP, uE3, hCG) or Quadruple Marker Test (adds Inhibin-A).
- Down Syndrome Profile: Low AFP, low uE3, high hCG, high Inhibin-A.
- Edwards Syndrome Profile: Low AFP, low uE3, low hCG.
- Genetic Sonogram (Level II Ultrasound): Detects structural malformations and soft markers, such as choroid plexus cysts for Trisomy 18, or atrioventricular canal defects for Trisomy 21.
Non-Invasive Prenatal Testing (NIPT)
- Mechanism: Analyzes cell-free fetal DNA fragments of placental origin circulating in maternal blood.
- Timing and Performance: Can be performed from 10 weeks onwards. It offers >99% detection rate for Trisomy 21 with a false-positive rate of <0.1%.
- Limitation: It is a screening test; positive results mandate confirmation via invasive diagnostic testing.
Definitive Diagnostic Methods (Invasive Procedures)
| Procedure | Gestational Timing | Tissue Sampled | Procedure-Related Risk |
|---|---|---|---|
| Chorionic Villus Sampling (CVS) | 10 to 13 weeks | Placental chorionic villi via transabdominal or transcervical route | 0.5-1.0% risk of pregnancy loss |
| Amniocentesis | 15 to 20 weeks | Amniotic fluid containing desquamated fetal amniocytes | 0.1-0.3% risk of pregnancy loss; represents the gold standard |
| Cordocentesis | > 18 weeks | Fetal blood from the umbilical vein | Slightly higher than amniocentesis; used for rapid karyotyping |
(Data derived from)
Laboratory Techniques for Fetal Tissue Analysis
- Karyotyping (G-Banding): Involves culturing cells and staining metaphase chromosomes. It detects all numerical abnormalities and structural translocations but requires 10 to 14 days for cell culture.
- Rapid Aneuploidy Detection (RAD): Includes Fluorescence In Situ Hybridization (FISH) and Quantitative Fluorescent PCR (QF-PCR). Delivers results targeting chromosomes 13, 18, 21, X, and Y within 24 to 48 hours.
- Chromosomal Microarray (CMA): Compares fetal DNA against a reference genome to detect submicroscopic copy number variations. It is superior to karyotyping when structural anomalies are noted on ultrasound.
Comparison of Common Trisomies
| Feature | Trisomy 21 (Down Syndrome) | Trisomy 18 (Edwards Syndrome) | Trisomy 13 (Patau Syndrome) |
|---|---|---|---|
| Incidence | 1 in 700 to 1 in 1000 live births | 1 in 5,000 to 1 in 6,000 live births | 1 in 10,000 to 1 in 16,000 live births |
| First Trimester Profile | Low PAPP-A, High hCG | Low PAPP-A, Low hCG | Low PAPP-A, Low hCG |
| Key USG Markers | Thick NT, absent nasal bone, atrioventricular canal defects | Clenched fists, rocker bottom feet, choroid plexus cysts, polyhydramnios | Holoprosencephaly, postaxial polydactyly, facial clefting, omphalocele |
| Prognosis | Variable intellectual disability; long-term survival possible | ~90% mortality within the first year | ~90% mortality within the first year |
(Data derived from)
Genetic Counseling and Legal Framework
- Pre-Test Counseling: Differentiate between screening (probability) and diagnosis (certainty), outline procedure risks, and discuss potential incidental findings.
- Post-Test Counseling: Must remain non-directive. For non-disjunction trisomies, the recurrence risk in future pregnancies is approximately 1% or the maternal age-related risk, whichever is higher. Translocation-associated trisomies require mandatory parental karyotyping to accurately define recurrence risks.
- Legal Framework:
- Medical Termination of Pregnancy (MTP) Act (2021 Amendment) allows termination up to 24 weeks for significant fetal abnormalities, and beyond 24 weeks pending Medical Board approval.
- Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act strictly prohibits sex determination during any prenatal testing.