Residency · Residency · Medical Genetics Genomics
Prenatal Diagnosis of Chromosomal Abnormalities: Trisomy 13, 18, and 21
Overview
Trisomies 13, 18, and 21 are the most common autosomal trisomies compatible with live birth. All increase in frequency with advancing maternal age due to maternal meiotic nondisjunction. Each has a distinct clinical phenotype, natural history, and prognosis. Prenatal detection relies on screening (serum markers, ultrasound, cfDNA) followed by confirmatory diagnostic testing (CVS or amniocentesis). Counseling must be balanced, nondirective, and provide accurate, current information about the condition.
Trisomy 21 (Down Syndrome)
Epidemiology and Genetics
Trisomy 21 is the most common autosomal trisomy, with an incidence of approximately 1 in 700 live births. Ninety-five percent of cases result from free trisomy 21 due to maternal meiotic nondisjunction, 3-4% from Robertsonian translocation (most commonly rob(14;21)), and 1-2% from mosaicism. Among translocation cases, approximately 75% are de novo and 25% are inherited from a carrier parent, making parental karyotyping essential. Recurrence risk is approximately 1% for free trisomy 21 (above age-related risk), up to 10-15% if a parent carries a Robertsonian translocation involving chromosome 21, and nearly 100% if a parent carries rob(21;21).
Prenatal Ultrasound Markers
First-trimester markers include increased nuchal translucency, absent or hypoplastic nasal bone, tricuspid regurgitation, and abnormal ductus venosus flow. Second-trimester findings include echogenic intracardiac focus, shortened long bones (humerus, femur), echogenic bowel, mild ventriculomegaly, sandal gap toes, clinodactyly, duodenal atresia ("double bubble" sign), and atrioventricular septal defect. Isolated soft markers have low individual sensitivity, but combination of markers and integrated screening improves detection. Structural anomalies, especially AVSD, increase likelihood significantly.
Postnatal Features and Management
Characteristic features include flat facial profile, upslanting palpebral fissures, epicanthal folds, Brushfield spots, small ears, and hypotonia in infancy. Intellectual disability is typically mild to moderate (IQ 40-70). Congenital heart disease occurs in approximately 50% (AVSD most common, also VSD, ASD, TOF). GI anomalies include duodenal atresia, Hirschsprung disease, and tracheoesophageal fistula. Increased risks extend to leukemia (ALL and AML/TMD in neonates), hypothyroidism, atlantoaxial instability, obstructive sleep apnea, celiac disease, and Alzheimer disease (virtually universal neuropathologically by age 40). Life expectancy now exceeds 60 years with modern medical care. Early intervention, inclusive education, and health supervision per AAP guidelines are standard of care.
<image>Composite image showing common prenatal ultrasound findings in trisomy 21 including increased nuchal translucency, absent nasal bone, atrioventricular septal defect, and duodenal atresia double bubble sign</image>
Trisomy 18 (Edwards Syndrome)
Epidemiology and Genetics
Trisomy 18 is the second most common autosomal trisomy with an incidence of approximately 1 in 5,000-8,000 live births. Despite a higher conception rate, approximately 95% of trisomy 18 conceptions result in miscarriage or stillbirth. There is a female predominance among live births (60-80%). Over 95% of cases are free trisomy 18 from meiotic nondisjunction, with rare translocation and mosaic forms. Mosaic trisomy 18 has highly variable phenotype and generally better prognosis.
Prenatal Ultrasound Markers
Ultrasound findings include intrauterine growth restriction, clenched hands with overlapping fingers (index over middle, fifth over fourth), congenital heart defects (VSD, polyvalvular disease), omphalocele, choroid plexus cysts (nonspecific but suspicious when combined with other findings), single umbilical artery, clubfoot, micrognathia, strawberry-shaped cranium (brachycephaly with prominent occiput), renal anomalies such as horseshoe kidney, and polyhydramnios or oligohydramnios.
Natural History and Prognosis
Median survival is 5-15 days for live-born infants, with approximately 5-10% surviving to 1 year. Major causes of death include cardiac failure, apnea, and aspiration. All survivors have significant cognitive impairment. Ongoing ethical debate surrounds the intensity of medical interventions including cardiac surgery and resuscitation. Some families choose comfort care while others pursue interventional management. Counseling must respect parental values while providing accurate prognostic information.
Trisomy 13 (Patau Syndrome)
Epidemiology and Genetics
Trisomy 13 has an incidence of approximately 1 in 10,000-20,000 live births. Approximately 75-80% are free trisomy 13, roughly 20% result from Robertsonian translocation (most commonly rob(13;14)), and about 5% are mosaic. When a translocation is identified, parental karyotyping is essential to determine recurrence risk. Rob(13;14) carrier parents face an empiric recurrence risk of approximately 1-2%, lower than theoretical due to selection against unbalanced gametes.
Prenatal Ultrasound Markers
Characteristic findings include holoprosencephaly (alobar, semilobar, or lobar), midline facial defects (cyclopia, proboscis, cleft lip/palate often median), microcephaly, postaxial polydactyly, congenital heart defects (ASD, VSD, dextrocardia), omphalocele, renal anomalies (polycystic kidneys, hydronephrosis), cutis aplasia (scalp defect), microphthalmia or anophthalmia, and single umbilical artery.
Natural History and Prognosis
Median survival is 7-10 days, with approximately 5-10% surviving to 1 year. All survivors have severe intellectual disability. Similar ethical considerations as trisomy 18 apply regarding intensity of intervention. Mosaic trisomy 13 may demonstrate longer survival and variable phenotype.
<image>Comparison table of clinical features, ultrasound findings, and prognosis for trisomies 21, 18, and 13 including survival statistics and recurrence risks by mechanism (free trisomy, translocation, mosaicism)</image>
| Feature | Trisomy 21 (Down) | Trisomy 18 (Edwards) | Trisomy 13 (Patau) |
|---|---|---|---|
| Incidence | ~1/700 live births | ~1/5,000–8,000 live births | ~1/10,000–20,000 live births |
| Mechanism | 95% free trisomy; 3–4% Robertsonian translocation; 1–2% mosaic | >95% free trisomy; rare translocation/mosaic | 75–80% free trisomy; ~20% translocation; ~5% mosaic |
| Key US findings | Increased NT, absent nasal bone, AVSD, duodenal atresia, short femur | IUGR, clenched hands (overlapping fingers), CHD, omphalocele, CPC | Holoprosencephaly, midline cleft, polydactyly, CHD, renal anomalies |
| Serum pattern (1st tri) | ↓PAPP-A, ↑free β-hCG | ↓PAPP-A, ↓free β-hCG | ↓PAPP-A, ↓free β-hCG |
| Serum pattern (quad) | ↓AFP, ↑hCG, ↓uE3, ↑inhibin A | All four markers low | Not reliably detected |
| Median survival | >60 years | 5–15 days | 7–10 days |
| Intellectual disability | Mild-moderate (IQ 40–70) | Severe | Severe |
| Recurrence risk (free) | ~1% above age-related risk | Low (~1%) | Low (~1%) |
Screening Strategies
First-Trimester Combined Screening (11-13+6 Weeks)
This combines nuchal translucency measurement with maternal serum PAPP-A and free beta-hCG. Detection rate for trisomy 21 is approximately 82-87% with a 5% false-positive rate. The trisomy 21 pattern shows increased NT, decreased PAPP-A, and increased free beta-hCG. The trisomy 18/13 pattern shows increased NT with decreased PAPP-A and decreased free beta-hCG.
Second-Trimester Serum Screening (Quad Screen, 15-22 Weeks)
The quad screen measures AFP, hCG, unconjugated estriol, and inhibin A. Trisomy 21 produces low AFP, high hCG, low uE3, and high inhibin A. Trisomy 18 shows all four markers low. Detection rate for trisomy 21 is approximately 81% with 5% false-positive rate.
Cell-Free DNA Screening (10+ Weeks)
cfDNA offers the highest screening performance for trisomies 21, 18, and 13, with detection rates exceeding 99% for trisomy 21, over 96% for trisomy 18, and over 91% for trisomy 13. False-positive rates are below 0.1%. It remains a screening test requiring confirmatory diagnostic testing.
Integrated and Sequential Screening
These approaches combine first- and second-trimester markers for improved performance. Integrated screening reports combined results only after second-trimester markers are available. Sequential screening reports first-trimester results immediately, offering high-risk patients early diagnostic testing.
Confirmatory Diagnostic Testing
CVS (10-13 weeks) or amniocentesis (15+ weeks) provides a definitive karyotype. Chromosomal microarray provides additional information and confirms trisomy while identifying rare structural variants. FISH offers rapid preliminary results in 24-48 hours. Confirmatory testing is essential before any irreversible decision.
Counseling Frameworks
Nondirective Counseling
Counseling should provide accurate, balanced information about the condition including the range of outcomes. Exclusively negative language should be avoided; information about quality of life, available supports, and positive experiences of families should be included. Patient autonomy in decision-making must be respected. Contact with parent support groups and condition-specific organizations should be offered, and the emotional impact of the diagnosis acknowledged.
Decision-Making Support
Options after prenatal diagnosis include continuing pregnancy with preparation, continuing pregnancy with a palliative care plan (trisomy 13/18), or termination of pregnancy where legally available. Available interventions, surveillance during pregnancy, and delivery planning should be discussed. For trisomy 13 and 18, the range of approaches from comfort care to selected medical/surgical interventions should be presented without a single "correct" approach imposed. Multidisciplinary team involvement (MFM, neonatology, genetics, social work, palliative care) supports informed decision-making.
Emerging Considerations
Advances in medical care have improved survival in some trisomy 13 and 18 cases, challenging the historical "lethal" framing. A growing body of literature addresses parental perspectives and quality of life in children with trisomy 13/18 who receive medical interventions. Prenatal diagnosis discussions should acknowledge this evolving evidence and respect parental values.
<image>Integrated screening pathway showing first-trimester combined screening, second-trimester quad screen, cfDNA screening, and diagnostic testing options with detection rates and timing for each approach</image>
Clinical Pearls
Karyotype (not just CMA) should always be obtained for confirmed trisomies to distinguish free trisomy from translocation, as this determines recurrence risk and need for parental karyotyping. Robertsonian translocation carriers have significantly different recurrence risks than parents of children with free trisomy, making parental karyotyping mandatory when a translocation is found. Choroid plexus cysts in isolation are not an indication for invasive testing; they are common normal variants found in approximately 1% of mid-trimester ultrasounds. In trisomy 18, clenched hands with overlapping fingers is nearly pathognomonic and should trigger immediate further evaluation. cfDNA performance for trisomy 13 is lower than for trisomy 21, requiring counseling about higher false-positive and false-negative rates. Mosaic trisomies have highly variable phenotypes, and prenatal mosaicism findings require careful interpretation with experienced genetics professionals. The term "incompatible with life" for trisomy 13/18 is increasingly considered inappropriate; "life-limiting condition" is preferred.
References
- Bull MJ, Trotter T, Santoro SL, et al. "Health supervision for children and adolescents with Down syndrome." Pediatrics. 2022;149(5):e2022057010.
- Cereda A, Carey JC. "The trisomy 18 syndrome." Orphanet Journal of Rare Diseases. 2012;7:81.
- Patau K, Smith DW, Therman E, et al. "Multiple congenital anomaly caused by an extra autosome." Lancet. 1960;1(7128):790-793.
- ACOG Practice Bulletin No. 226: "Screening for fetal chromosomal abnormalities." Obstetrics & Gynecology. 2020;136(4):e48-e69.
- Janvier A, Farlow B, Wilfond BS. "The experience of families with children with trisomy 13 and 18 in social networks." Pediatrics. 2012;130(2):293-298.
