A caul is a piece of membrane that can cover a newborn's head and face. Birth with a caul is rare, occurring in less than 1 in 80,000 births. The caul is harmless and is immediately removed by the attending parent, physician, or midwife upon birth of the child.

An en-caul birth is different from a caul birth in that the infant is born inside the entire amniotic sac (instead of just a portion of it). The sac balloons out at birth, with the amniotic fluid and child remaining inside the unbroken or partially broken membrane.

A child 'born with the caul' has a portion of a birth membrane remaining on the head. There are two types of caul membranes, the first type of membrane is the inner layer called the amnion which is the amniotic sac that contains the fetus and the amniotic fluid; the second membrane is the chorion and is the outermost membrane around the fetus, it contains a complex series of blood vessels that are connected to the endometrium, which together with the chorion forms the placenta, the primary organ responsible for supplying the fetus with oxygen, nutrients and extracting waste.

The most common caul type is a piece of the thin translucent inner lining of the amnion that breaks away and forms tightly against the head during birth. Such a caul typically clings to the head and face but on rarer occasions drapes over the head and partly down the torso.

The caul is harmless and is immediately removed by the attending parent, physician, or midwife upon birth of the child. If the membrane is of the amniotic tissue, it is removed by easily slipping it away from the child's skin. The removal of the thicker membrane is more complex. If done correctly, the attending practitioner will make a small incision in the membrane across the nostrils so that the child can breathe. The loops are then carefully removed from behind the ears. The remainder of the caul is then either peeled back very carefully from the skin or else gently rubbed with a sheet of paper, which is then peeled away. If removed too quickly, the caul can leave wounds on the infant's flesh at the attachment points, which might leave permanent scars.

Birth with a caul is rare, occurring in fewer than 1 in 80,000 births. This statistic includes en-caul births, which occur more frequently than authentic caul births; therefore, authentic caul births are even more rare than indicated by the raw statistic. Most en-caul births are premature. Recent research has shown that surgical techniques during cesarean deliveries of severely premature infants that focus on delivering the infant with the caul intact can potentially increase the baby's chances of survival, as it protects the underdeveloped fetus from being harmed by the muscle contractions of the uterus, and from the doctors and nurses accidentally harming the fetus while removing it from the uterus.

According to Aelius Lampridius, the boy-emperor Diadumenian (208–218) was so named because he was born with a diadem formed by a rolled caul.

In medieval times, the appearance of a caul on a newborn baby was seen as a sign of good luck. 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.