The palm normally has three flexion creases. Sometimes, the two horizontal creases fuse to form a single crease, called a single palmar crease or a simian crease.
Many structures develop in the first few months of gestation .
Palmar creases develop early. Abnormalities in palmar creases may indicate the presence of other associated abnormalities. A single palmar crease or simian crease appears normally in approximately 1 out of 30 people. There may be other causes of a simian crease. This list is not all inclusive, and the causes are not presented in order of likelihood.