Although new mothers counting their newborns’ fingers may seem clichéd, in reality the finger count varies from the normal 10 digits more often than many realize. Syndactyly, characterized by webbed or conjoined fingers, occurs in approximately one out of every 2,500 births.
In most cases, syndactyly affects one hand and involves the middle and ring fingers. It is most commonly seen in Caucasian infants.
The hands of a fetus form between 26 and 54 days of gestation. Toward the end of that period, the hand is like a mitten. A cell signaling process occurs causing the webbing between the fingers to resorb and allowing the web spaces to form. In some fetuses, that signal to one of the web spaces doesn’t occur, resulting in skin and sometimes bones left conjoined.