You’d kind of think the letters would relate to St. Peter, being they’re near heaven’s gate.
maybe, but maybe not in a tramp-stamp way. St. Michael does a lot of things. He guards Eve's body, protects the Nile River, saves souls out from under Satan's nose. A knight in shining armour against evil and Satan. He's a healer, too, and so a protector of women.
Given the tatoo's location, which midwives would certainly observe on a regular basis, the tatoo could been a talisman of protection against death during childbirth, or maybe even a strong visual warning against sexual violation.