Classification of Polydactyly and Polysyndactyly

Polydactyly is the most common congenital hand anomaly, seen in approximately 1:700 pregnancies. It's classified according to the side of the hand or foot in which the extra digits arise.

Post-axial (most common): Extra digit(s) on the small finger or 5th toe side.
Central (least common): Middle three digits are involved
Pre-axial: Extra digit(s) on the thumb or great toe side

An extra digit consisting of soft-tissue without bone is usually an isolated finding without associated anomalies. An extra digit with bone is more likely to be associated with other anomalies. An autosomal dominant pattern of transmission is also associated with a favorable prognosis.

Post-axial polydactyly may also be associated with trisomy 13, Meckel-Gruber syndrome, Bardet-Biedl syndrome, Smith-Lemli-Opitz syndrome, short ribs–polydactyly syndromes, and Ellis-van Creveld syndrome.

Pre-axial and central polydactyly are more often associated with syndromes. Pre-axial polydactyly has been associated with Holt-Oram syndrome, short ribs–polydactyly syndromes, Carpenter syndrome, trisomy 21, VACTERL, and Fanconi anemia.

References

Rypens F et al. Obstetric US: Watch the Fetal Hands. RadioGraphics 2006; 26: 811-829.