In The Scarlet Letter by Nathaniel Hawthorne why is there a self inflicted scar on chest?

In the novel "The Scarlet Letter" by Nathaniel Hawthorne, there is no mention of a self inflicted scar on the chest of any character. Instead, the protagonist Hester Prynne is forced to wear a scarlet letter "A" on her chest as a mark of her sin and shame for committing adultery.