Trisomy 18 and 13

Definition

• Trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) are aneuploidies with an extra copy of chromosome 18 or 13 and multiple malformations.

Occurrence & epidemiology

Epidemiology

• Trisomy 18 occurs in about 4.1 per 10,000 pregnancies and trisomy 13 in about 1.7 per 10,000 (each including terminations and stillbirths).
• In trisomy 13 the live birth prevalence is about 1 in 10,000 to 20,000 and antenatal mortality exceeds 95%; in trisomy 18 fewer than 10% survive the first year, and girls are affected three times as often as boys.

Aetiopathogenesis

Aetiology

• In trisomy 18 more than 95% have full trisomy; the extra chromosome is almost always maternal, risk increases with maternal age, and translocations are rare.
• In trisomy 13 full trisomy is the most common form at about 80%; partial forms mostly result from a Robertsonian translocation t(13;14), mosaicism accounts for about 5%, and about 91% are due to maternal meiotic non-disjunction.

Clinical features

• Prenatally, trisomy 18 typically shows polyhydramnios, a small placenta, a single umbilical artery and growth restriction; after birth, clenched fists with the index finger overlapping the third and fourth fingers, microcephaly, severe heart defects (especially patent ductus arteriosus and ventricular septal defect) and anomalies of the diaphragm, abdominal wall and kidneys.
• In trisomy 13 midline defects are typical, such as holoprosencephaly, cleft lip and palate, microphthalmia and iris colobomas; 57–80% have severe heart defects, and polydactyly and genital anomalies are also common.

Diagnosis

• Prenatal suspicion arises from ultrasound findings such as growth restriction, from serum markers or from analysis of cell-free fetal DNA in maternal blood, the latter being a screening test.
• The diagnosis is confirmed by karyotyping and/or chromosomal microarray analysis of material obtained by chorionic villus sampling or amniocentesis.

Keep learning in the app

Further reading (selection)

1. StatPearls: Trisomy 13 (Patau Syndrome) (NCBI Bookshelf)
2. MSD Manual Professional Edition: Trisomy 18
3. MSD Manual Professional Edition: Trisomy 13

Cross-references