Trisomy 18 and Trisomy 13
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Direct answer
Trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) are autosomal trisomies that cause multiple congenital anomalies and severe intellectual disability. Most cases arise from meiotic nondisjunction, with maternal age as the main recognised risk factor, and karyotype confirms the diagnosis. Most affected infants die within the first weeks or months, so prognosis counselling and comfort-focused care form a large part of management.
What you must remember
- Trisomy 18: Edwards syndrome is the second most common autosomal trisomy in live births, after Down syndrome.
- Hands and feet in trisomy 18: clenched hands with overlapping fingers, and rocker-bottom feet.
- Face and head in trisomy 18: prominent occiput, micrognathia and low-set, malformed ears.
- Trisomy 13: Patau syndrome features holoprosencephaly, cleft lip and palate, microphthalmia, scalp defects and postaxial polydactyly.
- Heart and kidneys: congenital heart disease, most often a ventricular septal defect, and renal anomalies are common in both trisomies.
- Prognosis: most infants die within the first year, many in the first weeks, and only a small minority survive beyond infancy.
- Diagnosis: karyotype confirms the trisomy, while fluorescence in situ hybridisation or quantitative fluorescent PCR gives a faster result.
- Recurrence: the risk is low in most families, except when a parent carries a Robertsonian translocation.
A stepwise approach to a newborn with suspected trisomy 18 or 13
A newborn is born at term with a small body size, clenched fists, a heart murmur and a cleft lip. The workup separates the two trisomies and sets the prognosis.
- Recognise the pattern. Look for several anomalies across the face, hands, feet and heart, since an isolated minor defect rarely signals a trisomy.
- Differentiate by hands and face. Overlapping fingers and rocker-bottom feet point to trisomy 18, whereas holoprosencephaly, midline facial defects and postaxial polydactyly point to trisomy 13.
- Image the heart and kidneys. Echocardiography and renal ultrasound are needed early, because structural defects in these organs are common.
- Send the karyotype. A peripheral blood karyotype confirms the diagnosis, and rapid testing for chromosomes 13, 18 and 21 gives an early answer.
- Assess feeding and breathing. Feeding difficulty, apnoea and aspiration are frequent, so the team should decide early on tube feeding and respiratory support.
- Counsel the family. Explain the high early mortality, the low recurrence risk in most families, and the choice between active and comfort-focused care.
Where the exam tries to catch you
Three errors recur on this topic.
- Trap: choosing Down syndrome for a child with clenched hands and rocker-bottom feet Correct: these are classic trisomy 18 features, whereas Down syndrome shows a flat occiput, a simian crease and hypotonia.
- Trap: expecting a long survival in trisomy 13 Correct: severe brain and cardiac defects mean that most infants die within the first weeks of life.
- Trap: diagnosing trisomy 13 from a cleft lip alone Correct: a cleft lip is common in trisomy 13 but also occurs in isolation, so the karyotype is the basis of the diagnosis.
Frequently asked questions
What is the commonest cause of trisomy 18 and trisomy 13?
Most cases of trisomy 18 and trisomy 13 arise from meiotic nondisjunction, mostly in the maternal germ line. Maternal age is the main recognised risk factor. A smaller proportion result from a parental translocation, and mosaic forms can cause milder features.
Which prenatal ultrasound markers suggest trisomy 18?
Nuchal thickening, intrauterine growth restriction, clenched hands with overlapping fingers, rocker-bottom feet and congenital heart defects suggest trisomy 18 on ultrasound. A single umbilical artery is another recognised marker. These findings prompt invasive testing for a karyotype, because no ultrasound pattern confirms the diagnosis alone.
Why is karyotype essential in suspected trisomy 13?
Karyotype is essential because the features of trisomy 13 overlap with other syndromes, and only chromosome analysis confirms a full trisomy. It also shows whether a translocation is present, which changes the recurrence risk for the family. Rapid tests can give early results while the full karyotype is pending.
What is the prognosis for infants with trisomy 18 or trisomy 13?
Most infants with trisomy 18 or trisomy 13 die within the first weeks or months of life, and a small minority survive beyond the first year. Survival is shortest when severe cardiac or cerebral anomalies are present. Prognosis is therefore discussed early, and the management plan reflects the family's values and the infant's condition.