It formed when the man is embryo and it's a inherent fixed substance. The genetic flecker includes delomorphic speckle and invisible speckle.The delomorphic speckle is formed between 6 and 12 years old and get to top in 18 years old.The invisible speckle always appears on the face after the response of pregnancy.This is the reason why somebody distinguishes it between congenital freckle and postnatal freckle.

Actually,the incretion of pregnant woman can have a big change when she is childing,and that can stimulatethe hidden freckle to break out,so the freckle is not postnatal.The freckle is only arouse by genetic gene

The descendiblity of freckle have the characteristic of atavism,so that's why the parents have not freckle but the children have it.One or both parents with the lentiginose gene, but don’t have freckle on their face, so the gene has been transmitted to their children.