Trisomy 13 (Patau Syndrome; Trisomy D) A chromosomal disorder resulting from an extra chromosome 13, producing many developmental abnormalities, including severe mental retardation and forebrain abnormalities. Trisomy 13 occurs in about 1/10,000 live births; about 80% of cases are complete trisomy 13. Advanced maternal age increases risk, and the extra chromosome is usually maternally derived. Midline anomalies are characteristic. Gross anatomic defects of the brain, especially holoprosencephaly (failure of the forebrain to divide properly); cleft lip; cleft palate; microphthalmia; colobomas (fissures) of the iris; and retinal dysplasia are common. The supraorbital ridges are shallow, and the palpebral fissures usually are slanted. The ears are abnormally shaped and usually low-set. Deafness is common. Infants tend to be small for gestational age. Simian crease, polydactyly, and hyperconvex narrow fingernails are common. About 80% of cases have severe congenital cardiovascular anomalies; dextrocardia is common. Other midline defects include scalp defects and dermal sinuses. Loose folds of skin often present over the posterior aspect of the neck. The genitalia are frequently abnormal in both sexes; cryptorchidism and an abnormal scrotum occur in the male, and a bicornuate uterus occurs in the female. Apneic spells in early infancy are frequent. Mental retardation may be present.