Patau synrome, also known as trisomy 13, is a chromosomal abnormality, a syndrome in which a patient has an additional chromosome 13 due to a non-disjunction of chromosomes during meiosis. Some are caused by Robertsonian translocations. The extra chromosome 13 disrupts the normal course of development, causing the characteristic features of Patau syndrome. Like all non-disjunction diseases (Down syndrome, Edwards syndrome, etc.), the risk of disease in the offspring increases with maternal age at pregnancy, with about 31 years being the average. Patau syndrome affects approximately 1 in 12,000 live births.