Normally the baby presents the crown (top) of its head in the opening of the uterus during birth, with the neck bent and the chin on the chest. This lets the smallest diameter of the head pass through the birth canal. In a very small number of cases, the neck becomes extended (bent back) instead of flexed (bent forward), and the face presents itself to the outside. This is a significant problem, as in a face presentation the largest diameter of the head is trying to force its way through the birth canal. The result is a very long labour, and damage to both mother and baby is possible.
Obstetricians can sometimes disengage (push up) the head from the pelvis and bring it back down again with the crown of the head presenting, but in most cases a caesarean section is the treatment of choice.