One milestone in the development progress of children is the development of a birthmark.

That statement is false

Birthmarks usually caused by overgrowth of blood vessels, smooth muscles, or fat. It appear shortly after birth and most of the time really harmless.

The thing is, not every new born child will get a birthmark so it could not be categorized as a milestone for children development

Depression is lifelong, though you can get medication to with it. the difference between sadness and depression is that sadness lasts a little bit, depression last a long time and it goes on and off. people usually get depression through experiences they have been through, or genetics.