Polydactyly is the presence of an additional digit. This digit may range from a fleshy nubbin to a complete digit with controlled flexion and extension. It may be unilateral or bilateral and occur in either or both the hands and feet. A well-developed metacarpal or metatarsal bone may also be present. Polydactyly can be classified as postaxial (the most common variety), preaxial and central. Postaxial polydactyly occurs in the ulnar side of the hand or fibular side of the foot, while preaxial refers to the opposite side (radial and tibial side, respectively). Central polydactyly consists of complete cutaneous syndactyly between the long and ring fingers with an extra digit hidden within the web (also known as syndactyly type 2 or synpolydactyly). Polydactyly is a very common condition that can present as an isolated birth defect or part of a syndrome, skeletal dysplasia or chromosomal abnormality with males being more commonly affected. The majority are isolated conditions with an autosomal dominant mode of inheritance. When part of a syndrome, the mode of inheritance tends to be autosomal recessive. A triphalangeal thumb may be seen in cases of preaxial polydactyly.


One or more extra digits can be seen in both longitudinal and cross-sectional scans. This diagnosis has been made occasionally in the late first trimester with transvaginal ultrasound as well as embryoscopy. Polydactyly of the hand is easier to diagnose than that of the foot. A small fleshy nubbin can be more easily overlooked than a normally ossified extra digit. When positioning of the fingers does not permit a definitive diagnosis, detection of an additional metacarpal or metatarsal is helpful. Its absence, however, cannot exclude polydactyly. There are varying degrees of development and motility of the additional digit/s, and they may often be inserted at abnormal angles. An attempt should be made to identify syndactyly, as this may be helpful in the differential diagnosis. Partial or total fusion of the phalangeal diaphysis and/or epiphysis can be sometimes seen at more advanced gestational ages.

Differential Diagnosis

See Associated Syndromes for a list of conditions with polydactyly

Sonographic Features

Normally developed or hypoplastic extra digit

Normal or abnormal angle of insertion of the extra digit

Normal to absent movement in the extra digit

Presence or absence of an additional metacarpal or metatarsal

Triphalangeal thumb

Cutaneous or osseous syndactyly

Associated anomalies may be evident

Associated Syndromes

  • Acrocallosal syndrome
  • Bardet-Biedl
  • Carpenter (acrocephalopolysyndactyly)
  • Elejalde
  • Ellis-van Creveld
  • Grebe
  • Greenberg-Rimoin skeletal dysplasia
  • Greig (cephalopolysyndactyly)
  • Hydrolethalus
  • Jeune asphyxiating thoracic dystrophy
  • Kaufman-McKusick
  • Klippel-Trenaunay-Weber
  • LADD (Levy-Hollister)
  • Meckel-Gruber
  • Miller-Dieker
  • Oto-palato-digital syndrome type II
  • Pallister-Hall
  • Pfeiffer
  • Pseudotrisomy 13
  • Short rib polydactyly (Types I-III)
  • Simpson-Golabi-Behmel
  • Smith-Lemli-Opitz
  • Townes-Brocks
  • Trisomy 13


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