When the head no longer recedes between contractions this indicates that it has passed through the pelvic floor and that delivery is imminent. The maximum diameter of the head (biparietal) stretches the vulval outlet and there is no recession even after the contraction passes off. This is crowning of the head.
Laceration of the perineum often occurs during birth of the head. By the time the head begins to appear at the vulva.
At this stage the midwife or doctor must control the head to prevent its being born suddenly and it must be kept flexed until the largest diameter has passed the vulval outlet.