Trisomy 13 syndrome: The presence of three copies of chromosome 13, rather than the normal two. Children with trisomy 13 syndrome are profoundly mentally retarded and have multiple malformations, commonly including scalp defects, hemangiomas (blood vessel malformations) of the face and nape of the neck, cleft lip and palate, malformations of the heart and abdominal organs, and flexed fingers with extra digits. The majority of trisomy 13 babies die soon after birth or in infancy. Also known as Patau syndrome.
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