This condition is called hexadactyly. The word hexadactyly literally means six digits. In medical usage, hexadactyly does not specify whether the six digits are fingers or toes (although in Greek "dactylos" is without equivocation a finger).
The 6th digit can be located in three different locations: on either side of the extremity or somewhere in between. With the hand for example, the extra finger can be out beyond the little finger (which is called ulnar hexadactyly) or out beyond the thumb (radial hexadactyly) or, finally, between two of the normally expected fingers (intercalary hexadactyly).
Far and away the most frequent form of hexadactyly is ulnar (postaxial) hexadactyly. Next comes radial (preaxial) hexadactyly. And far and away the rarest form of hexadactyly is intercalary hexadactyly.
Hexadactyly in itself can be innocuous, absolutely harmless and very easily remedied, when the hexadactyly is an isolated finding and the baby is otherwise entirely normal. Ulnar hexadactyly with just a rudimentary tag of a sixth digit, for instance, can be very simply treated by tying it off with one suture.
Hexadactyly can be seen on some prenatal ultrasound scans. To present a real case, an ultrasound scan showed "a rudimentary 6th digit on both hands." A more detailed ultrasound confirmed "a 6th digit on ulnar side of each hand" and showed that 6th digit "is small and just a floppy skin tag." No other morphological (physical) abnormalities of the fetus were visible. Another ultrasound with even greater resolution revealed "a rudimentary digit/skin tag on the palmar surface of the hand lying between the 4th and 5th digits." Again, no abnormality was seen elsewhere on the baby.
The differential diagnostic list of disorders causing hexadactyly is pretty long. But many were excluded by the absence of other malformations. It also seemed a particular type of hexadactyly. If it were an intercalated extra digit (with the extra finger situated between the other fingers), the list would be much smaller and only a very few entities such as the Pallister-Hall syndrome and Greig syndrome (but there would usually be other things to see in either of these 2 syndromes). The final diagnosis in this case has not yet been made. We have present the case to convey the quandary that can occur when hexadactyly is discovered antenatally (before birth) today.
Hexadactyly is the most frequent form of polydactyly, a diagnosis that encompasses all cases of extra digits, irrespective of the number of extra digits in a particular case.