Trisomy 13 (Patau syndrome)

The incidence of trisomy 13 is about 1 per 15 000 live births.
The majority of trisomy 13 conceptions spontaneously abort in
early pregnancy. About 75% of cases are due to nondisjunction,
and are associated with a similar overall risk for recurrent trisomy
as in trisomy 18 and 21 cases. The remainder are translocation
cases, usually involving 13;14 Robertsonian translocations. Of
these, half arise de novo and half are inherited from a carrrier
parent. The frequency of 13;14 translocations in the general
population is around 1 in 1000 and the risk of a trisomic
conception for a carrier parent appears to be around 1%. The
risk of recurrence after the birth of an affected child is low but
difficult to determine. Prenatal ultrasound scanning will detect
abnormalities leading to a diagnosis in about 50% of cases. Most
liveborn affected infants succumb within hours or weeks of
delivery. Eighty percent die within 1 month, 3% survive to 6
months. The main features of trisomy 13 include structural
abnormalities of the brain, particularly microcephaly and
holoprosencephaly (a developmental defect of the forebrain),
facial and eye abnormalities, cleft lip and palate, postaxial
polydactyly, congenital heart defects, renal abnormalities,
exomphalos and scalp defects. Survivors have very severe mental
and physical disability, usually with associated epilepsy, blindness
and deafness.