When a baby develops in the womb, at first the hands and feet form in the shape of a paddle. Then, around the sixth or seventh week of pregnancy, the hands and feet begin to split and form fingers and toes. In the case of webbed fingers or toes, this process is not completed successfully, leading to digits (fingers or toes) that are fused together or webbed.
In most cases, webbing of the fingers or toes occurs at random, for no known reason. Less commonly, webbing of the fingers and toes is inherited.
Webbing can also be related to genetic defects, such as Crouzon syndrome and Apert syndrome. Both syndromes are genetic disorders that can cause abnormal growth of the bones in the hands and feet.