From Wikipedia:

caul or cowl (LatinCaput galeatum, literally, “helmeted head”) is a piece of membrane that can cover a newborn’s head and face.[1] Birth with a caul is rare, occurring in fewer than 1 in 80,000 births. The caul is harmless and is immediately removed by the physician or midwife upon delivery of the child.

In medieval times the appearance of a caul on a newborn baby was seen as a sign of good luck.[6] It was considered an omen that the child was destined for greatness. Gathering the caul onto paper was considered an important tradition of childbirth: the midwife would rub a sheet of paper across the baby’s head and face, pressing the material of the caul onto the paper. The caul would then be presented to the mother, to be kept as an heirloom. Some Early Modern European traditions linked caul birth to the ability to defend fertility and the harvest against the forces of evil, particularly witches and sorcerers.[7]

Folklore developed suggesting that possession of a baby’s caul would give its bearer good luck and protect that person from death by drowning. Cauls were therefore highly prized by sailors. Medieval women often sold these cauls to sailors for large sums of money; a caul was regarded as a valuable talisman.[8]

In the Polish language, the idiom w czepku urodzony/a (‘born in a bonnet’), and, in Italian, nato/a con la camicia/a (‘born with a shirt’), they both mean a person who is always very lucky.

Not all cultural beliefs about cauls are positive. In Romanian folklore, babies born with a caul are said to become vampires upon death.[citation needed]