It is lethal in almost all cases by the age of 6 months. It results from an extra chromosome 13 secondary to nondisjunction or translocation.

These individuals sometimes complete fetal development and are born, but are severely mentally retarded, and physically malformed. Patau's syndrome that arises because of this can be inherited. Patau syndrome is also known as trisomy 13, because the baby has an extra chromosome 13 in all or some of their body cells. The karyotype here demonstrates trisomy 13 (47, XX, +13) also known as Patau's syndrome.

Trisomy 13 syndrome (formerly Patau's syndrome), 47,XY,+13. There is a full set of 23 homologous pairs, plus an extra chromosome 13.

In a further 1 in 20 cases, only some cells have the extra copy of chromosome 13. Trisomy 13 is typically due to having three full copies of chromosome 13 in each cell in the body, instead of the usual two copies. Patau's syndrome that arises because of this can be inherited. Babies with trisomy 13 have many abnormalities, involving nearly every organ system in the body, as well as developmental delay. Lab technicians compile karyotypes and then use a specific notation to characterize the karyotype. Patau syndrome, or Trisomy 13, is the least common of the autosomal trisomies and most severe, after Down syndrome (Trisomy 21) and Edwards syndrome (Trisomy 18). It is rare for babies to survive for very long if liveborn because of the multitude of anomalies that are usually present. Trisomy 13. Trisomy 13 Syndrome is a rare chromosomal disorder in which all or a portion of chromosome 13 appears three times (trisomy) rather than twice in cells of the body. This notation includes the total number of chromosomes, the sex chromosomes, and any extra or missing autosomal chromosomes. : break: 5qter -->5p15: For example, because an extra 21st chromosome is the cause of Down's syndrome, write the notation of the karyotype as "47, XY, +21,Trisomy-21, Down's Syndrome." The syndrome is associated with advanced maternal age. Lalani, in Cardioskeletal Myopathies in Children and Young Adults, 2017. In a UK-based study from 2003, 44 cases of trisomy 13 and 88 cases of trisomy 18 were examined.

This is known as trisomy 13 mosaicism. For example, 47, XY, +13 indicates that the patient has 47 chromosomes, is a male, and has an extra chromosome 13. Occasionally, only part of 1 chromosome 13 is extra (partial trisomy 13). The extra copy of chromosome 13 in Patau syndrome causes severe neurological and heart defects that make it difficult for infants to survive. Consult a doctor or diagnostic manual to learn the names of disorders caused by chromosome irregularities, and write the disorder's name in the notation to make it more complete. In some affected individuals, only a percentage of cells may contain the extra 13th chromosome (mosaicism), whereas other cells contain the normal chromosomal pair.

Greater than 90% of fetuses with trisomy 13 have findings detected on prenatal ultrasound. A few may also have translocated chromosomes.