Cauls

Cauls are pieces of membrane that can cover a newborn's head and face at birth. They are rare, occurring in fewer than 1 in 80,000 births. Cauls were sold by midwives to seafarers as they were thought to protect against drowning.
Through the 14th-17th century some people thought the caul was formed when intercourse was had late in the pregnancy, to protect the infants' heads.

Here's a seafarer. He probably bought a caul.